I think what he's asking is: if he shoots with one crossbow, drops it, and quickdraws another with the intent to use it as an improvised weapon, can he take an AoO with the second crossbow?
Which I suppose is the same question as: if he shoots with a crossbow, drops it, and quickdraws a sword, can he take an AoO with the sword?
The question being, when you find yourself armed with a different weapon at the end of the round than the one you performed your attacks with that round, can you make attacks of opportunity with it?
My answer would be no. Again, this is because of the shield bash rule. You're already spending the round attacking with one weapon. That's your weapon for the round. If you could bash with a shield, drop the shield, and somehow (quickened Drawmij's Instant Summons?) grab another shield as a free action, you still couldn't use that new shield for AC. So you shouldn't be able to perform that trick with a weapon either.
edit:
Come to think of it, I would probably make an exception for thrown weapons. If you've been throwing daggers all round, and end the round with a dagger in your hand, then fine. You can take an AoO. But if you end the round with a sword or improvised weapon? No dice. The point here being that the "I shoot and then draw as a free action" manoeuver is an exploit to get around the restriction of using a crossbow as a missile weapon and a melee weapon in the same round. It doesn't matter if it's a new crossbow or the old crossbow. You still can't use it as a melee weapon if you shot this round.