Well, if the front guide needs replacing then the cylinders will have to come off so you can accurately center the new guide when installing it. If not centered, or crooked, the chain will run sort of diagonally across the guide and chew the edges up. Here's some old guides that weren't centered properly .....
When you install a new guide, you need to center it in the cam chain tunnel. You do that by measuring between the side of the guide and the side of the tunnel where the guide sticks out of the cylinder, and you must do this at the top and the bottom. Obviously, you couldn't check the bottom with the cylinders still installed .....