FWIW (I don't do this for a living):
There are a couple of ways to get the correct installed key...from a system...I suppose that most users settle on one.
I use SIW Download - http://www.gtopala.com/siw-download.html
, which will reflect accurately license keys for Windows, MS Office and other programs. These will be under the Licenses category.
The SPs themselves are only significant if you intend to try a repair install or want to run sfc /scannow to try to repair damaged/missing Windows files. These tasks can only be accomplished properly by ensuring that the disk used is at least as current as the installation.
In other words, a slipstreamed XP SP3 disk...should be able to be used on any system with the same version of XP...for repair purposes (since SP3 is the most current SP for XP).
Icrontic » Blog Archive » Slipstreaming Windows Easy - http://icrontic.com/articles/slipstreaming_windows_easy
I guess that I see no need to change any product key at all, unless you intend to install XP on a system where the key info is unavailable, for whatever reason. The key for the system...if correct...will work with a different SP level for that operating system.