Well... you're trying to replace the glass in the first place, so it's technically already destroyed.

Also, I broke my Lumia 520 screen and I'm waiting for the replacement screen to arrive. If you use your thumbnail or something very thin but strong, you can separate the glass from the frame without causing more damage to the glass by running it underneath the edges of the glass, all the way around.

Granted, it took a couple of goes to separate them because of the glue, but it did come off without breaking either the frame or the glass.