I'm not sure what happens to a map importing it into UV Mapper. My experience with the free version is it is extremely friendly and doesn't muck with a properly-made map. On the other hand, there are several things Carrara does that are not friendly towards maps. There are a lot of rough edges in how the built-in UV mapping functions work. For one example, if a map that has a contiguous mesh split out into several non-contiguous UV regions will sometimes, brought into Carrara, try to re-weld itself across those seams. For another example, Carrara's own cylinder mapping is actually incorrect; it wraps one polygon behind the rest of the mesh instead of splitting at the seam. It also has a nasty habit of developing disconnected or broken points that can, if messed with, cause Carrara itself to crash. However, in general, you should be able to switch back and forth between those applications without having your maps scrambled. But then, my experience is with Carrara, the freeware UV Mapper, and Poser. Between those I can generally maintain the maps.