Transformation in Fantasy is easy. 'Poof' and it's done, or alternately one may need to utter a mystical incantation such as 'Hsif a tweN ekaM'. In Science Fiction it is much more difficult, even when one takes Clarke's Third Law -- "Any sufficiently advanced technology is indistinguishable from magic" -- into account. Thus, though my personal transformation was more of the fantastical 'poof' type, I offer a selection of possible and not so possible technological transformation methodologies for your writing pleasure.

1: Exterior Transplants

This is going on currently in a limited manner and involves plastic surgery and surface transplants to transform one. Currently this is limited to modifying the appearance of human flesh and transplanting artificial substances such a fishing line to form cat whiskers. However, it is possible that eventually surface transplants could include such things as genetically modified fur (to be grafted on in pace of skin), either artificial cybernetic or genetically engineered eyes (for enhanced vision), and a tail. Of course, sticking extra bits on is easy; getting the human brain to actually make use of the new bits is a lot tougher. The human brain is a badly understood miracle of biology that barely manages what it does. MRI studies of animals suggests that changes in senses would require a corresponding change in the brain to process the new information. Dogs have large chunks of their brain dedicated to processing smell; thus, for a human to have a canine-equivalent sense of smell, the brain would have to be re-ordered. Even controlling such things as a tail (allowing conscious control over its movement) or retractable claws would likely require neural re-engineering which is likely to be very difficult, if not impossible.

2: Interior Transplants

Although this does not allow a physical transformation, which is what most fans of transformation literature seem to prefer, it does allow interesting story ideas. More efficient organs could be genetically created and transplanted, allowing longer breath-holding, better physical performance, or even seeming miracles. Different lungs could be transplanted to allow breathing of alternate atmospheres, such as a partially terraformed Mars. Alternately, transplants of unmodified, 'vanilla' organs could extend life, suggesting a rush on organs for transplants. Organs could even be replaced with cybernetic devices allowing even further enhancements, such as stored air for hours (just hope nothing punctures the tank). It might even be possible to graft cybernetic storage devices onto the brain, allowing eidetic memory or implanted skills, knowledge, or even personalities. Individuals could have a permanently implanted cybernetic AI as a personal aide/confidant to search the nets for information, to remind of schedules, to talk. Or to enable external control of the physical form, thus making the biological brain a helpless prisoner in its own body.

A note on brain modification: At this point we don't understand how the brain works, other than in the most general sense. We know how neurons work, but what is mind? What is self? What is sentience? One theory is that an individual develops by trial and error, trying various combinations like a neural net until skills are learned, such as communication. According to this theory, my consciousness thinks of something, and that thought is translated through a trial-and-error-taught filter to a sequence of appropriate words, which is then spoken. Your physical body hears the word, and it is passed through your own trial-and-error-taught filter(s), until finally your consciousness interprets the information. Though studies have confirmed that common functionality seems to occur in the same gross areas of the brain from one individual to another, the specifics may be unique from one individual to another, which would make any kind of direct neural interface different from individual to individual. In other words, it's not as easy as cyberpunk makes it out to be.

3: Brain Transplants

There is nothing theoretically unlikely about taking a brain, and transplanting it into a new body. And, if the body happens to have been genetically engineered and grown in a vat, it could be just about anything desired. The problems with this, though, are first, how to control features of the new body (particularly senses or limbs) which did not exist in the brain's original corpus, and second, making sure that the brain actually fits into the new body's cranial cavity. No normal-sized raccoon transformations by this method. If the brain is better understood, it might be possible to restructure the brain as required for a new body, or one may be forced to grow neural clusters to control new features, allowing the actual brain only the grossest access to the new limbs. For example, a tail controlled in this manner would literally have a mind of its own. It might react to hormonal changes within the body (such as anger), or might continue oblivious to how the owner feels. It might also be difficult or impossible to link a brain to a new nervous system, which, if true, suggests that one might have to transplant the brain and the nervous system. This could be done by suspending the body in a tank of nutrient fluid, and releasing a virus or nanomachines which consume everything except nervous tissue and brain tissue. It could be possible, if nervous tissue could be grown, to transplant a brain into a new body, grow hard wired though untrained neural structures to control new limbs/senses, and have the brain's consciousness learn to use the new equipment.

4: Remote Control

If a direct data link to a brain can be achieved, either by a surgical procedure, or by a slow creation of a unique interface through trial and error by nanomachines, it would be possible to remotely control a properly equipped host body via a kind of telepresence. One could be put into an isolation chamber, and have the sensory input of an appropriately wired creature such as an animal, or another sentient, sent to the isolated body. If both bodies are equipped, it might be possible for one body to give commands remotely to the other body, or possibly each could control the other body simultaneously (insert some kinky sex ideas here). Likely though the control would be somewhat limited, and likely the remote body would feel at least somewhat 'numb'. However, this could be overcome if enough bandwidth is available.

5: Virtual Bodies

An alternate method, if a direct neural link can be achieved bypassing the senses, would be for the host to control a virtual body within a virtual environment. Within such an environment, anything would be possible as long as the data stream could contain the information, including instantaneous transformation. However, the problem of extra limbs would still exist. They could be controlled by coded macros, but would be unlikely to be directly controlled by the host mind.

6: Mental Transference

A sufficiently advanced technology might be able to isolate the consciousness within a biological brain and copy it, either onto cybernetic hardware, or into another biological brain. As only the consciousness and memories would be copied, a new body could have wiring for new limbs/senses -- assuming the existing consciousness could be mapped onto a new neural structure -- and new limbs/senses could be controlled, although their use would have to be either learned, or else implanted through 'fake' memories/knowledge created artificially. The one problem with this is that the brain is continually changing, and thus the brain would have to be frozen at one point for such a copy to be made. This could be done by a 'safe' freezing, or might require the individual to be killed, and then the brain sliced and copied before it begins to decay. And in this case, if something goes wrong, you are dead.

7: Retroviruses

There has been a lot of talk about viruses written to infect cells and then rewrite the genetic code within the individual cell. At first glance, this suggests that it might be possible to actually have a body transform gradually over time, but the problem is that most tissues have long since quit growing in the relevant sense. In particular, bones would not change. This method could be used to modify soft tissues, including such things as hair colour and skin colour, and could allow the growth of fur, scales, or feathers over the skin. Given the invariance of the skeleton, however, the body's overall anatomy would have to remain human.

8: Nanomachines

This is the other 'magical' technology that a lot of writers use. Nanomachines are devices that are equivalent in size to molecules that would be injected into a body and rebuild cells and organic structures to a new model. There are many problems with this, not the least being what happens to the heat generated, and why the person doesn't die as their body explodes in seemingly uncontrolled cancer. If the nanomachines modify the brain chemistry, then likely the individual would be destroyed, creating a new brain that could learn to be another person. In a very controlled fashion, limited physical changes could be done over time, as bones were lengthened, muscles lengthened, etc., but it would have to be done one step at a time and very carefully, and the head would still have to remain human sized to hold the brain.

9: Biogoo

Given a truly complete understanding of the brain, it might be possible to write a retrovirus, or program nanomachines, to rebuild the body into a non-differentiated biology with a distributed neural processing network that is structured to recreate the individual brain. This 'biogoo' could shape itself into different forms by limited cell differentiation, thereby allowing actual transformations, though it is likely that a great deal of time would be required to change from one form to another. The individual would likely have to cocoon him/herself and spend weeks or months dissolving the old body and growing the new.

10: Genetic Engineering

Alert readers have probably realized that many of the problems listed above are derived from having to preserve the subject's individual mind/identity during the transformation. There is nothing wrong with creating a new life form and letting it develop its own mind and consciousness, but this is not really a transformation of an individual, more a transformation of the race.

Doubtless I have missed methods, but hopefully there's a lot to think about here. Use this list for ideas or not, as you please, but don't let it limit you.