While I am in complete agreement with all the suggestions (keep model clean and consistent), I understand that there are cases of course, when one needs to quit the "rules". So ThomThoms UV toolkit will do the job for you - even keeping all the UV co-ordintes.
uv-toolkit.png
Also, there are a bunch of "reverse faces" plugins that will do the same. FrontFace is by Tomasz and while it is reversing the faces (even through group and component wrappers), it copies the material (until then applied to the back face) onto the front face - also keeping UV's.