I'm sorry if this has come up before. It seems to be one of those things that would be a FAQ but I didn't find it in the unofficial FAQ, nor in the most recent Errata.
If you use the Summon Agent card and replace a key monster (For example, Rolok in Archive of Arrizon) in a quest, what happens? Heroes immediately win? Quest unwinnable by heroes if the monster in question is not part of a group that can be Reinforced? Neither seems likely, and I will be playing this as "This is not permitted to happen" but I'm curious to know how this is normally handled, if at all.
The Plot cards that summon an Agent only allow replacing an open group so it is not a problem.