Hi,

For the future, it's not a good idea to type in all caps. First it is not so easy to read, secondly it is considered the 'netiquette' equivalent of SHOUTING.

I have no idea if it is possible to change the product key on an installed system.
However, there's two observations.

1. You don't need that product key if the existing system is updating OK. It will only be needed if you have to reinstall XP.

2. If the 'legit' copy is corporate licence and you have it for personal use, then it's not really a legitimate copy. You could well find that, if you install it, Microsoft will identify it as an illegal installation.