Aeropac has two kinds of motor retainers. There are glue-on retainers for motors from 29mm to 75mm diameter and bolt-on retainers for motors from 54mm to 98mm diameter. To swap a bolt-on retainer to a different rocket would only require a second set of anchors to embed into the aft centering ring of the second rocket.
First, I did my Level 2 with the same rocket as my Level 1. You just need to ensure it can handle the additional forces and speed. Most rockets with a 54mm or larger motor tube should be able to if built correctly.
Second, there is a difference between the adapters and retainers. You will need a retainer for each rocket (altho the screw-on part is interchangeable). The Adapters are I guess what you would call interchangeable. I use the same 75mm - 54mm adapter when I'm adapting from 75mm as well as 98mm (they nest into each other.)