My friend at Virgin Alaska tells me they're required to get first class on DH longer than 5ish hours.

At my current regional we're not guaranteed anything in particular other than a passenger seat. Crew scheduling, or whatever system they use, usually initially puts you in a crap seat near the back. 24 hours prior I'll check in on the app and can usually move up to a premium economy or at least exit row seat. Is it the same at Spirit, where you check in on the app and then can move your seat? It also puts us on the upgrade list but I've gotten that exactly twice.