Incorrect. There are no options in the game to disable the expansions. There are a couple of ways to do it though.
The problem you have is any items you found that were added to the game in Forgotten Gods just don’t exist in AoM. And iirc correctly the game reverts to an older copy of your character which is why the swords were in the stash and not on your character.
The best thing to do is make another character which you only use for multiplaying with your friends and keep the other for solo play.