@Anonymous
You kind of did this in reverse of what is optimal. Specifically, it is easier to unwrap the lower poly model and then add a turbo smooth or openSubdiv on top of Unwrapped model for the higher poly version.
...but when I reduce the polygons the uvs are transformed, what can I do?
How exactly are you reducing the polygons? Any time you make changes to the mesh (deleting or adding edges for example) of an Unwrapped model, you run the risk of breaking the UV Mapping. It might be pretty similar, but it will not be a perfect match. You can manually tweak the UV Unwrap of your lower poly version to match the corresponding parts of the model texture on the higher poly version by displaying the texture map in the Unwrap UV Editor window and comparing your results to the high poly version in your viewport. It will take some patience, but it is achievable. That way the same texture will line up the same on each version of the model.
But in the future, note that it is a lot easier to do the UV Unwrapping on the a lower poly version of the model.
