Are grains interchangeable?

Are commercial grains within a particular manufacturer, size, and propellant, interchangeable? Or, specifically, If I have a 75mm 4 grain CTI Blue Streak reload, and want to fly it as a 3 grain motor, could I get a 2 grain reload of the same and have 2 x 3 grain reloads? Do grains work that way, aside from the XL grains?

It would almost certainly have to be flown as an experimental motor at a TRA launch. Assuming the core diameters are the same, and it's not some sort of different geometry (moon burner, end burner, etc), they should be the same. However, what WILL almost certainly be different are the nozzles and liners. You could probably cut down a 4 grain liner to 3 grains, but you wouldn't be able to add more liner to your 2 grain reload (you'd have to get a whole new one). Also, they will almost certainly have different nozzle throat diameters, so they will reach different internal pressures. At best you'll end up with a slightly different (and probably less efficient) motor, at worst you'll overpressurize your casing. I wouldn't recommend it.

Ah.. Of course there is more engineering than meets the eye. I had not thought about nozzles, that makes sense. Thank you.

Won't say where it was done or who did it, but I've seen it done successfully under the auspices of "research"...many times. It was cool. Never cutting liners, but just swapping grains. I'd love to see the resulting thrust curves

The last EX N motor I made had a two-piece liner and flew just fine, FYI.