I have 5 of the 6.4:1 pro qualifiers and I am wondering if I can buy a 7.1:1 replacement set and switch out the gears in one of them to make it a higher speed reel?  does anybody know if the gear sets are interchangeable?

Thanks.

In the same reel they should be, but then again I'm not a reel expert. Mike over @Delaware Valley Tackle should be along any minute to give you the definitive answer. 

You'll need to purchase the main and pinion gear from a 7:1 PQ to do the swap. It's the same reel other than that. 

Yes the hears are interchangeable. And yes pinion and main are installed as a set. In general I discourage gear swaps to accomplish what is commercially available, but if want help with the swap let me know.