I ended up having XP on 2 separate drives on one of my older machines. I just removed the cables from my old HD 1st, then put the cables on the brand new 2nd (Larger) HD and loaded XP. When I was done with the install I plugged in all the cables to both HD's making my new one the primary drive. After it boots and before XP loads it brings up a screen where it lets you select which one you want to start up with.
It "as others have already said" leaves the original copy as is and just adds a new drive letter for the added drive.
The only downside was when I finally removed the old smaller HD it would still ask which one I wanted it to start in even though there was only one left. Never did figure how to get rid of that request.