Thanks for the assist, everyone. I think I was confused by the 30-hour message that shows when you try to change your seat assignment. Once I'd reached the 24-hour mark for both flights, I was able to check-in without a problem.

Certainly had an interesting return: 7+ hour delay, 3 of that onboard the ERJ at ROC, with the EWR storms. And the final indignity, a 30-minute wait at EWR at 1:00a for the GA to move the jetbridge. Ouch, it was a painful day trip!