I am getting a sniper for airsoft, but my shot speed limit is 350 fps. Could I buy a sniper shooting at 450 fps and lower it to around 350 fps?
If so, how could I do that without cutting the spring?
Yes, you can change the speed, there are a few ways to do this.
You can change the weight of the BBs you use. The heavier the BB, the lower the speed. Please keep in mind most fields do not measure by speed in fps but by energy in joules, as the "force" or how much the BB will hurt, is dependent on joules not fps, so check where you're playing to see what their limit is.
You can change the inner spring, obviously the weaker the spring the lower the speed in fps, please don't cut your spring; that will ruin it. Just buy a lower power one, they go by units of measure for example m110 meaning a velocity of 110 metres per second using 0.20g BBs.
I personally recommend doing both, as the heavier the BB the less the wind and air pressure will affect it equating to a more accurate shot, but changing the BB weight will not change the force exerted by the BB so you should also change out the spring if you want it to hurt less.