UV room is an extension of Paint Room, use it to tweak model’s UVs previously baked in Paint Room.
UV room and Retopo UVs are not connected on any way, they are total different meshes inside 3DCoat, one is called Retopo Object and the other is called Paint Object.
Once something is in Paint room, the Paint Object cares only about what Paint, Tweak and UV rooms do.
So, to Update Paint room objects UVs, you use the UV room, once you hit the “Apply UV-set button” then it’s done, you don’t have to do anything else, you will paint, see and use the new UVs, even if you already have paint on it, 3DCoat will automatically re-project the paint layers to the new UVs, so it should look almost the same before you did any UV change.
Also, as a side note, If you need to or want to update the Paint Objects with the Retopo Object for whatever reason, like if you make a change on the topology and decided to create new UVs and all inside retopo room.
You can use Bake menu and use “Update Paint mesh with Retopo mesh“, which replaces currently visible object in Paint room or “Retopo -> Per Pixel Object (no baking)” which doesn’t replace anything and creates a new Paint Object along with the ones you had before.
Importing into Second Life
If you are bringing your mesh from other 3D software and changing the UVs, then you have to make sure you are importing into Second Life the mesh with new UVs you export using Export Objects and Textures (for Second Life, don’t forget to check Z-up coordinate system so it will be oriented correctly when you import it).
Tutorials
How to swap UVs: This video covers one easy method in 3DCoat, to modify a model’s UV’s after doing some texture painting work….without any penalty.