The kind of mission you fly and how you swap the planes seems to play a role, at least according to findings posted by others.
Example:
You select a mission in the QMB (or the single missions section if you set it to allow you to customize the mission) and swap a flight for Spitfires for a flight of Blenheims.
The Blenheims still keep the Spitfire AI and they try to fight with you.
On the other hand, changing a flight of bombers into fighters results in passive fighters (good for target practice this one
).
I haven't really tested this myself but it was reported by a few different posters on these forums a bit after the EU release.