I maxed out a gun at 2500gp to repair ( scar-L ) and if you want to save your GP, just max it like that, and dont repair it, just go into a game throw it down, and pick up another, it wont go past its max , once its damaged all the way, it will not damage any further. (but don't try to use it for more than 2 shots, the recoil puts it shoootng straight above you, its sick.
EDIT: dam, I totally msunderstood the question, I reread it and, dam, way off, should be a differnet gun, therefore , diff stats. IE new damage bar, if it were the same gun, I assume it would keep.