Switching elements doesn't cause loss of rank, only switching trades. Transfer from Res to Reg is not a remuster either, even if you wind up in a differnet element. For example, if you decided to CT to Reg as a Res Int Op, you would submit your request, your file would go before the merit board for selection, and your qualifications would be taken into account. It may be determined that you don't need a portion of the 5A, but if you wanted to wear, say, a Navy uniform (or if the selection decided they needed more Navy Int Ops) you may skip te common phase, and only have to do the Navy Environmental phase.
Keep in mind that everything should be prefaced with "may" and "might." There are no guarantees.