Characters can be re-apped as if they're fresh characters, if the player wishes to change the loss or just reset. It'd be treated the same way as a normal reapp -- drop the existing character, and send an email at any time letting us know you'd like to reapp and what you're resetting. As long as there's space on your requested team(s) they'll be placed right away.

If you're just switching for a different character entirely, you'd have to wait until the regular app round, unless it's someone else you've played before and can reapp.