I agree with Sheepdog. I did exactly that with a Remington 1100 Standard frame 20 gauge. I didn't want to cut the original stock, so I found one for a 12 gauge to cut down and install a Kick-EEZ pad. Now my wife can shoot it with the pad, and shorter LOP, and I can shoot it if I re-install the original stock.
I wouldn't worry about collector value. If you bought it as a safe queen then you wouldn't be considering the pad and you would not have bought one with a scratch. When I purchase something that I want, of course I try to get a good deal and not pay a ridiculous price, but to me, there is also value in having something I enjoy. I don't think twice about dropping a little more money in to it to make it even better in my eyes.
When I bought the 1100 Standard it was about $350. I then put time and money in to it to refinish the wood myself, say another $50. I then bought another stock for $75. Then I bought a new recoil pad for about $50. So I have about $525.00 in to the gun. Do I ever think I'll get that back if I sell it? Nope, but I have a gun that is super fun to shoot and a gun that is comfortable for my wife to shoot. A day at the sporting clays course with her, and no kids (LOL), is worth more then that to me. Oh, and I am also considering getting it re-blued. That will really put me in the hole! Still, doesn't matter to me.