This was my guess, too. At least on the girls side, a year doesn't seem to mean much for olders.I disagree. If trapped players are good they would be on the top team already. A few months don’t make that much difference.
Then I looked at the rosters for a few top teams. 18-20 kids on each, but only 2-4 of them with the later grade year.
We'll see some turnover, but not a complete overhaul. I'm sure there are about ten parents of trapped RL players for every one kid who actually moves up.