All the prices i've seen are pretty much the same online but i'll be buying from Amazon.
You will be able to buy the upgrade edition of Premium or Ultimate to install over XP MCE.
Differences between retail and upgrade:
Retail can be installed on any PC providing is it is installed only on one PC at a time.
Upgrade can only be installed on a PC which already has pre-installed one of the qualifying operating systems.
This page describes which versions can be upgraded: click here