What’s the latest an alternate can get upgraded and still leave with everyone else?

I'm an American first-time applicant and I've been made an alternate for this year and as far as I know, alternates can get upgraded and leave with everyone else as long as it happens within some timeframe before the regular shortlisters leave. Otherwise, you leave sometime between after everyone else leaves and December.

Does anyone know/have experience with how late this can happen before they send you separately, arriving in Sept. or later? I also heard this is done in waves, with at least 1 wave leaving with everyone else due to people dropping/rejecting before then. I ask because if I don't get upgraded before then, I'd honestly rather just wait until the next application cycle and roll the dice again.

EDIT: Clarification