Epics and regular issues are treated very differently by Jira.. these differences should be accounted for and managed when one performs a "move" action on a story to convert it to an Epic.
Epics cannot be assigned to sprints from the backlog view. Stories can. When a story that's in a sprint is converted to an Epic, it should be removed from the sprint.
Epics don't show up in backlog view (they have their own side panel) so if a story is converted to an epic while part of a sprint, you don't see it as part of the sprint until the sprint is started when it shows up on the sprint board.
Long story short, my team converted a bunch of big stories to epics and they made a mess of our sprint when I started it because they didn't show up as being part of the sprint in the backlog view.. the move process should have removed the sprint identifier.. or i dunno, shown up as part of the sprint in the backlog view.
