Here is an interesting question that has come up in one of the games I run.
“If a character is paralyzed and cannot use her legs, would polymorphing into another creature ,even if it is another bipedal creature, enable the character to walk?”
The answer is, 'It depends'.
If the paralyzation is the result of magic, then the answer is definitively 'No'. A curse follows you whereever you go until you break it. Try to wiggle out of a curse by changing form will certainly fail.
If the paralyzation is the result of physical trauma, then the answer is 'maybe' depending on the rules you use to establish what being paralyzed means and the specific rules you use for shapechanging.
In my game, there are specific disadvantages like 'Lame' or 'One Eye' you can take during character creation. Those disadvantages specifically mention that shapechanging cannot cure the defect. Whatever you change into will suffer an equivalent effect. They are presumed to be the result of curses or similar powerful magic.
Paralyzation through physical trauma is expressedly equal to permanent dexterity and/or strength drain in my game. Hense, shapechanging only 'cures' it if it can provide sufficient increase in dexterity or strength to ensure mobility. Whatever you change into will carry your strength and dexterity drain with you, which may result in limited mobility - effectively you went from being unable to move to gimped. (Which makes me think I should implement some natural encumbrance rules, now that I think of it.)
There is some sound basis of this in mythology, and I generally agree with S'mon. Disfigured shapechanging magical beings would always reveal their true identity because, regardless of what form that they changed into, they would be forced to include some sort of disfigurement. For example, the witch who shapechanged into the form of a beautiful maiden, would be forced to conceal a wart or grosteque mole or other give away of her true nature. Fairies could change their form at will by use of their glamor, but would always have some defect (which varied from fairy to fairy) such as six fingers or feet being on backwards. Odin gave up his eye to the Norns, and changing his appearance and form couldn't fix that. Likewise, Oberon was short, and could not by altering his form appear as other than short.