You can try to call in at 48 hours before departure to see if you can get that seat, or it should be available at T-24 hours at OLCI. If it isn't available, select the best alternative but so long as you don't print or App your boarding pass then you can change it if it frees up in the final 24 hours.

If when you call at T-48 hours the agent can't change it, that may well be correct, it may be held for other passengers being shuffled around, but you can call later on, it may get unblocked at any point thereafter.