Doing a downgrade on a spring in a gearbox isn't too difficult. Watch a youtube video, follow along, and you should be able to do it without much problem.
...Maybe do that once you've got a backup gun, though.
Most people seem to run a separate gun for indoor vs outdoor (that's what I usually do), both to save work on downgrading the gun, as well as for the gun's size. For example, it's not as much fun to go around a corner with a full length M4, as it is to come around with something like a P90.
There are some AEGs with quick-change, or easy-change springs or upper half of the gear box, but those aren't very common.
Gas blowbacks (GBBRs) have a functional bolt, similar to a real gun, that can adjust their velocity - either by modifying or swapping a valve, or loosening/tightening something for a quick adjust. Or, you can just have a separate bolt that you drop into your gun. I use the separate bolt method as well.
So in summary, the easiest and most common thing to do is to have a separate gun for indoors and outdoors - both to save the work of swapping the spring at the beginning and end of every season, as well as to have two different form factors in your weapons.