I actually spoke with HS Precision the other day and they were a big help. Basically, the factory butt spacers/pad is glued on with epoxy. HS recommended a couple of ways to get them off: heat gun and a utility knife, or a bandsaw, followed by a belt sander. There are no screws involved.
I started off with a utility knife and when I got close to the stock, I started with the belt sander. When the sander got the pad pretty hot, it started to melt the glue and peeled off. Just needed one more hit to clean the glue off the stock.
I used epoxy to attach the new pad, ground it down, ready to go. It was pretty easy once I kind of knew what I was trying to do.