Upgrading Windows.. don't want to lose subscription on multi PC licence

Ok, i have a 3-PC key for my NIS.  I am only using 2 at the moment.  I am upgrading the OS in my laptop from XP to Vista.  I am afraid when i go to type the password in that it will simply use my third key instead of the second one that it was using. Then i will not have the third key avaliable anymore.

 

What can i do to prevent this?