Too Cold to Release Monarch Butterflies?
If you no longer have high temps in the 50's F, your monarchs won't have a very good chance to join the migration. Still, some release on the best day possible to give them a chance...even if it's a slim one. You could also keep them indoors and feed them over winter.
They might survive a few weeks up to a few months, but they'll come out of diapause (sexual immaturity) and will likely die before spring. I would give them a chance to fly, if at all possible....good luck!
A second alternative is finding a local butterfly exhibit or a university entomology dept that might be interested in studying it. good luck!