it's from what I know never been an issue in the past.
I've upgraded nt 4.0(OEM) -> win2k(retail)
XP HOME(OEM) -> XP PRO (retail)
and XP HOME(OEM) -> Vista Home Premium(retail)
Never tried to upgrading from Vista(OEM) to anything but considering the past it's not likely it will be an issue for you. However I always strongly encourage everyone to do a clean install (Vista upgrade discs can be used to install the OS even without a previous installation all you need is the serial of the previous OS)