Only some barrels need to have the window enlarged. by far and large I think most aeg barrels don't need to be touched. The hop material is just cut to match the existing window.
To reverse it, you just remove the r-hop material and scrape off the glue you used to secure it to the barrel and replace your bucking with one that does not have a shaved mound (hur hur hur, that's right, I said it)
I am trying to determine if the hopup arm needs to be modded or not. I think it does not have to be, you can use a standard cylindrical nub, but using the block nub provides even more stability. Obviously you have to alter the arm if you want to use the block for full effect, so if you install one with a modded arm, you will need to replace the arm if you ever revert. And of course... the only thing you need to replace in the whole rig as maintenance would be the shaved bucking as the hop material is reputed to never wear out.