[Hectic]

Depending on the gun will make a difference but yes you will nust need to change the spring if it is an aeg.
Once u change if down to 350ish (if that is the cqb limit in bc) u can just leave it like that. 400fps for outdoor isnt a goal it is a limit. U can play outdoors efectively with 350fps and 400 wont make u more effective.
As for how easy is it. It isnt all that hard to swap a spring but it can be dificult depending how handy u are (im a prety handy guy overall but the time i tryed to rebuild a gearbox i did a horrid job of it so i havent tryed it again) id suggest a gun doc do it for u. Once it breaks then is a good time to try your hand at it because heck its alrdy broken so u may as well try. But there is no point tryin to swap the spring on yer own and messin up on shimming or somethin and wind up strippin gears or piston teeth or who knows what else.