You'll likely get an initial small drop, 10-15 fps (ish) going from a brand new spring to a used spring having had a fair few hundred cycles. Personally I prefer a worn in spring as it will be more consistent.
If FPS drops a little too much you can either swap for a new one which is a little stronger or drop an "adjuster ring" in to increase pressure and boost up by 10fps.
The only reason to swap is if it becomes damaged or collapses entirely (AirSoft Pro ones used to do this).
Spring quality will affect the FPS drop, if it's a good quality spring (Rapax for example) it will likely be truer to the desired output and stay there consistently more than a really cheap spring which is badly made and will likely deform and cause problems.
The worst spring I have ever had is the Sniper One branded piece of garbage from ASPUK. Supposedly an M150 which only performed at 470 initially and then dropped off from there, deformed after around 6 months of use. Swapped said spring for an AA M150 and the difference was night and day, actually got to 490FPS and it stayed there.
