"Upgrade" path from Windows 7 RTM 90-day trial to full version

"If this is the Windows 7 Enterprise 90-day trial you're talking about, you're going to need to reinstall after the trial period is up. From Microsoft TechNet:

The 90-day Trial is the full working version of the Windows 7 Enterprise,
the version most of you will be
working with in your corporate
environment. It will not require a
product key (it is embedded with the
download).

The 90-day Trial will shut down once every hour when you have reached the end of the 90-day evaluation period.

The 90-day Trial is offered for a limited time and in limited quantity. The download will be available through March 31, 2010, while supplies last.

After the 90-day Trial expires, if you wish to continue to use Windows 7
Enterprise, please note that you will
be required to purchase and perform a
clean installation of Windows 7,
including drivers and applications.
Please keep this in mind; Windows 7
Enterprise is not available through
retail channels."