Mostly they just need new seals and putting back where they were. Occasionally the nylon tip on the pressure adjuster will need facing, then the reg needs to be set up again.
Worse case scenario is having to start from scratch with a reg that need both a rebuild and pressure setting, that's when a reg pressure checker is seriously useful, but even then, especially with Air Arms reg, you can end up with a little reg creep. This usually takes only a few moments to fix, but needs the air tube emptying and refilling once again.
I know AA regs reasonably well, they are pretty easy to get into, once you have them off the rifle (that can be the hard bit) but other regs can be more difficult just because of how tight they are screwed together.
"I'm your huckleberry, that's just my game"