No, new install of XP to go with the new version of MC. I presumed that the logic would be to read the e-mail address from the on-line form and work out if a purchase had been made under that address. Obviously not (!) Is there a way around this (apart from installing 11, restoring the licence, upgrading and then removing 11)...
The short answer is no.
To get the upgrade price you have to have a registered copy of MC11 installed to upgrade from.
You don't actually have to do the removing of MC11, the installation of MC12 will give you the option and remove it for you if you wish.
You could then save the key code, uninstall everything, reinstall MC12 and unlock it with the new key code.
MC12 will not be affected if MC11 is also installed, as a matter of fact, MC11 and 12 are designed to co-exist on the same computer.