Genetic Modification
DNA (deoxyribonucleic acid) is a phenomenally complex molecule found in the chromosomes of all life forms. Put very crudely, it's a chemical code that defines a life form. It is unique for every individual. Given its key role in defining a life-form's appearance, you could perhaps be changed into the animal of your choice by its modification.
The difficulty is the sheer complexity of the material involved. If DNA is a code, we are far from cracking it yet. However, when we do know what makes DNA 'tick', we will be able to use it. Great advances have been made in this direction, but we still do not know enough. It is possible that not all information that controls the form of an organism is in the DNA. Much more research needs to go into just how the shapes of cells and groups of cells are determined.
Replacing all the human DNA with other DNA would not work. You would not then begin to transform. You would only be yourself, but with different genetic material. While new cells might be of the new form rather than human, this is more likely to lead to lethal cancers than changing shape.
More success might be achieved if a study of metamorphic life-forms was conducted. A tadpole's transformation into a frog involves a far more radical alteration of shape than a man into most of what we want to transform into. Suitably engineered DNA could achieve this change, if it could be introduced in a harmless fashion - we can hardly open up every cell in the body and physically replace the DNA. This might also be helped along with small surgical procedures in order to make the transformation faster or better controlled. We might place special 'marker' cells where we want a new limb to grow, for example. This would make the transformation easier to engineer.
One solution for the delivery method might be an artificial virus. A virus works by mutating a cell's DNA to produce more virus cells instead of new host cells. If it could be made to change resident DNA into 'New Improved Metamorphic' DNA, and when the virus could be somehow guaranteed to infect almost every cell in the body, this would be a neatly effective form of transformation. Mass deficiency would have to be made up for (If your chosen form is larger than a human, that is). Perhaps by vastly increased appetite during the transformation. Enveloped viruses which don't destroy a cell when exiting from it would be the best choice.
The research done with this method could be sold at a very hansom price to medical firms. That money would make life easier for our newly transformed. (It may also help cover the probably extensive cost of creating very large amounts of custom viruses.)
This method might also be dangerous to the world. If we weren't careful we could easily create a new virus that for example, attack the nervous system. These dangers would have to be closely guarded against. We would have to use precautions against mutations and contagious strains.
