How can i disconnect part of mesh, reconnect it to another part and make it a part of different UV island? I was modifying a boot model consisting of several meshes and ran into a big problem. I have two main mesh parts: the sock (top image on the left) and the boot (top image on the right). I need to disconnect a part of sock (bottom image on the left) and reconnect it to the boot mesh. I tried doing this by hitting the Y short key but this creates additional vertices which messes with the export script. So i need to disconnect and reconnect without changing the total vertex/edge/face count.. I also need the repositioned bit to become a part of a different UV island (bottom image on the right - i sort of drag-and-point where i want the piece to move to). Again, i can't unwrap because blender creates a totally different layout.. I'm almost positive that it's just impossible to do all that but maybe i'm missing something. I've spent 6 hours modifying the model before i realized there was a problem...
This is old question but I'd figure I'll post possible answer anyways because someone else might google this. 

I'd just pin (select these verts and press p) the vertices in uv editor that need to stay put. Leave all seams unpinned. Then just press u and do basic uvunwrap the whole mesh. Only those vertices that you wanted to move should move. If you get weird result then you probably have a missing seam, wrong vertice pinned, hidden geometry in edit mode or something else. After the uvunwrap press alt-p in uv editor to get rid of the pin tool.