In order for Disney to loose GotG in Orlando, the team would literally have to join the avengers. As in, be a part of the main group. And even then, it would likely have to be more than just a single movie. As in, they would have to join them in the comics as well.
Universal has rights to a very specific set of characters and their immediate universe. The caveat that seems to be catching people is a footnote that covers teams, such as Avengers and X-Men, where by if a character joins one of those teams, then they have the option to use the character.
This does not mean that simply having a character in the same movie or even comic as a team gives Universal these rights. The short of it is GotG can crossover with the Avengers with no problem as long as they aren't suddenly dropping their own team name and claiming to be members of the Avengers now. And even then, I'm not sure a movie is enough to put that exception into play. After all, these rules were drafted around the comic book universe, well before there were any movies to base these rules on.