The issue is with the decision, so asking "Is it moral?" is a potentially misleading framing because of the connotations of "moral" that aren't directly concerned with comparing effects of alternative actions. So the choice is between the scenario where a person made promises etc. and later stuck with them while miserable, and the scenario where they did something else.

Suppose you became deeply religious as a young adult and married someone of the same religion with a traditional promise to be loyal to them until death. Divorce was unthinkable to your spouse and you had repeatedly reassured them that you fully meant to keep your promise to never leave them, no matter what changes the future brought. You are now no longer religious and remaining married to this person makes you miserable in ways you are sure you can't fix without betraying who you currently are. Is it moral to leave your partner? Why and why not? (Don't worry, this is a hypothetical situation.)