Umm... iirc they changed it a while ago so that you cannot change/unequip items while flying. (so that archers couldn't fly and punch someone in the air causing them to die painfully)
It's possible the 'fix' was only client side though which would mean that a proxy or similar program would allow you to still change items in the air... it just seems unlikely though.
Dumb thought but what if you used the alternate equipment system...
Sorry I can't be of more use but I don't really play current co... Hell I haven't since before BP. As far as I knew though you couldn't fly with weapons anymore.