Give it some time, and you will run into computers where your OEM copy of, say, XP will work on some computers, but not all. There is nothing worse than going to activate and you don't have the right license key, what you're doing here is making a disc that KNOW will install correctly, AND have no problems at the activation point. So, every disc you get your hands on (even the Vista Anytime Upgrade disc) you must make an ISO of it, and label it correctly (i.e. XP Pro OEM, XP 64 bit Retail, etc.) so you don't get confused, as an OEM disc is absolutely necessary to complete this project. Trust us. In answer to your most recent questions:
1: Quite difficult, albeit possible. OEMScan is your answer, check the MSFN forums. (Seem to recall Nlite being able to do those though, hmmm.)
2: You cannot "upgrade" a license key, it will only be valid for that specific flavor of windows. You cannot even downgrade it.