I am confused. (which is pretty normal )

There are two options:

1.  remap A to use B map , and therefore textures.

2. Modify the texture for B to fit the uv map of A .

remap A is a matter of using a modeller or uv mapping utility and moving all the uv coordinates to the new positions which is simply very tedious because theres lots of them.

Modify the texture will depend on just how far out they are , but might be attempted in photoshop with some cutting, pasting and scaling , and maybe liquify to move stuff around.

There is a texture conversion utility but it needs its own template for the conversion process.