Upgrading Vista Retail Version

I just bought a HP Pavilion a6750f, 8 GB RAM. 750 GB harddrive, quad-processors, ATI 2600 pro graphics card, etc... It came with 64 bit Vista Premium installed. I have a 64 (and 32 bit) Retail version of Vista Ultimate I want to install it (that came from old computer). I asked HP about ugrading to my retail version and they advised against it strongly. This Vista Ultimate version ran great on old computer (where XP was pereviuosly installed). Why would HP not want me to install retail 64 bit version of vista ultimate on this new computer which more than enough computer to handle the OS. I understand possible driver issues and know pretty much how to handle all that. I would like to install the 64 bit ultimate but am unsure after talking with HP. Does anyone have suggestions? I have already made the one complete backup of the 64 bitvista premium installed. I'm not gonna to instal Ultimate to hopefully you guys give me some advise.

Thanks

Thanks for the reply. I been thinking about it and thats about what I came up with (the same solution). You know, I had this same vista Ulitnate on another HP computer and they were kind enough to give support but, their support was never that great anyway . I must say though, HP support is the best I ever delt with which is probably not saying much is that about the only computer support I've ever dealt with (besides Dell which was terrible).
Thanks Again for the Reply as this is the only Forum I have ever been to besides just googling problems I've had in the past... Ohh yeah,, I contacted microsoft and, hell, I'm a amature and knew more than they did.. Its crazy! Just being honest...