Do you mean you want to change the stock spring in a TM VSR? If yes you need to see if your cylinder is the earlier type with a pin in the side (That ha to be carefully drilled out) or the newer version which cannot be opened as it's crimped. If it's the earlier then a stock vsr can run approx 400fps with just a spring change. If you add a type B hope lever you should find it's lift heavier BB's, although sub 500 a type A might be better as the power/weight ratio means .3-.4 would be a better option. If you're after looking at turning it into a 500fps beast then ideally you'll be upgrading the cylinder,head, spring, spring guide, trigger (Including sear) and swapping the hop arm to a type B.
Check put ASPUK as they're the most reasonable cost wise although the company has just been taken over and stock is low.
I know guys that have just changed the spring, spring guide, sear and hop arm and it works fine but remember the stress on that sear/trigger will evebtually cause a failure. But if funds are tight it's a great way to start!