You cannot change conflicts in the middle. Once you’re committed, you’re committed. Your only other option is surrender (see Surrender on page 75).
“Thus adventurers may only flee from an attacking dragon–since they cannot kill, drive off or capture it.” –Asymmetric Conflict Goals, page 149.
If your players lack the Might for a particular type of conflict, they cannot engage in it. If they attempt something the rules say they can’t, tell them. “It’s too much for you.” You’ll note that the only monster in the book that doesn’t list a Flee conflict is the Lizard Man, who is Might 3…the same as adventurers.
If your players have the Might to engage a monster, but face some other restriction–it can only be killed by fire or a magic weapon or silver–that’s a different story. They can defeat it, they just can’t kill it. A spell might allow the players to enter a Kill conflict with a troll, but unless they set fire to it afterward, it will be back the next night. If they defeat a werewolf in a Kill conflict but don’t have silver weapons (or magic weapons or something), they drive it off when they win. Better find a way to kill it before it returns.