I've never seen it done. Once a player has played matches, I don't believe that he can be removed from you team. I don't know if they can make a exception on the 14 roster limit, but it couldn't hurt to ask. I know that there is a deadline for adding new players to you roster.
Depends on what you mean by immediately. From my experience you can play down in the Fall league, which is really part of the '07 Championship season. Any other play you must continue at you End of Year rating. I was a 4.0 last December ('05), the early start ratings bumped me down to a 3.5. I played the Fall league as a 3.5, but had to finish the year at 4.0 in tournaments.