I believe they are listed in numerical order, so you should be able to redefine them and reassociate them. This will work for starting a new design. if you are tryign to do this with an existing design then it is probably a lot of cutting and pasting and maybe not worth it.
Note that redefining the standard layers in PADS is probably going to break something that got hardcoded somewhere... Good luck.
John- Yeah, that's was I figured. I didn't see any way to do it.