Stretch is only available in advance to Summit members (not Ascent) and to people who buy the Classic Plus fare at frontierairlines.com. So it's not likely to be full at 24:01 before departure time. However if you want to do the paid upgrade at check in, it might be safest to upgrade a online check in 24 hours ahead.

I think there's a good chance that at airport check in there may be Stretch available, too. But I'd say your odds of having it entirely pre-reserved is rather small.