The main packet difference is in inventory related stuff in game. other than that 90% of all the packets are identical between irose and evo
the files for your GUI are all found in 3ddata/control/res inside the client's VFS.
The XML to display them all is in 3ddata/control/xml in the client's root folder (NOT VFS)
the cut scenes such as teh animation as you fly into the char select area is in 3ddata/scripts (VFS) It's all written in LUA which is editable in a text editor.
Some files have different structure in evo STBs are teh main difference. Most other files in the VFS use exactly the same format so if you are a bit clever with figuring out what controls what, you can make any char select work in any rose version





