I cant find this answer any where.

Ok I have a copy of XP Home (same copy on 2 computers 1PC, 1 Laptop) and XP x64 on a 3rd PC. I plan to buy Vista Ultimate so I can do the family licensing deal. my 2 PC's will run vista fine my old lap top with XP home on it will not run Vista (Dell 5000e)

If I buy Vista Ultimate (upgrade) will I still be able to use my old XP or will the Key become blacklisted? Or do I need to buy the “Full” version to keep my old xp keys valid.