I was never quite satisfied with shapeshifting as presented in the Amber rules. After much thought, I decided to have two distinct (at least at low point levels) forms of shapeshifting. Because of their basic take on the universe, I call them "scientific" and "magical".
Scientific shapeshifting is what the rules describe. A good (if extreme) example would be the morphing Terminator in Terminator II. Flexibility of form is very high. Even those with just a single template will have a good deal of variety possible in that area. For instance, if you have avian template, you can shift to any bird form which has approximately your mass. The more advanced practitioners can control the arrangement of their internal organs and features.
Conservation of mass is the major disadvantage of scientific shapeshifting. You can fudge a little, but losing or gaining more than a third of your mass is impossible.
Magical shapeshifting is closer to traditional lycanthropy. A good example is the wizards' duel in Disney's Sword in the Stone. Notice whether Merlin is a mouse, a goat, or a crab, he is blue, has bushy white facial hair, and eyeglasses.
Conservation of mass is laughed at; any size from mouse to elephant is easily assumable. Correspondingly, the flexiblity of forms is reduced. The scientific avian shapeshifter can assume any bird form in the proper mass range: if assuming an eagle form, he can be fat, trim, long, short, etc. The magical avian shapeshifter has only one eagle form, sort of his ideal inner eagle. In exchange, he ignores mass, and can switch to anything from a sparrow to a roc.
For the magical shapeshifter, there is a sort of conservation of features. It is required that the he have at least one visible distinguishing feature. No matter what form, all distinguishing features remain (transformed appropriately). Wounds count as distinguishing features as well.
At very low levels, the distinction is almost irrelevant. At very high levels of shapeshifting, it definitely becomes irrelevant. But in between, it is interesting.