I'm unaware of one, and there may in fact be no way to do this. I'm curious if there's a way to quickly change items around in a slot. ie: say you're an infiltrator and have the REK in your only hand slot. Is there some quick method to swap this with, say, an AMP? Would be nice if you could setup your Fx keys to do this, but it doesn't seem possible. It can be slow and clumsy to do this real fast via the inventory window (it's not uncommon for me to drop something when I do this quickly).

So is there anything?

If you right-click on the item in your inventory it will go to an appropriate holster.

What if there is an item already in the holster? Will the items just swap spots? That's what I'm really after.

Yes, it swaps in that case.