Replacing the gas block is fairly easy.
An AGR will simply allow control of bolt speed and slow the unlock/extract/eject portion of the cycle to ensure a more reliable cycle and reduce wear and damage on the internal parts of the gun.
Given the dimensions of the selector in a second generation AGR, it will have some effect on all operating systems, but will work better on some then others. Exactly which ones is as yet to be determined stateside. HK knows.
With Spring style roll pins, it's suggested to replace rather then re-use.