Unoffical upgrade Windows 7 RC Ultimate to Windows 7 RTM Enterprise

"Use easy transfer to back up your data and do a clean install
What you're trying to do is unsupported by Microsoft and I'm sure they made it hard to do.
On top of that your changing the software version, which won't go well probably:

Windows 7 Enterprise can only be
activated with a Volume License MAK or
KMS key if it is not activated against
a local KMS server. Enterprise
requires a VL KMS/MAK out of the box
or activation against a local KMS
server. It can be rearmed up to 3
times.

How-To-Geek also wrote a post about doing what you're asking but in the end he notes:

There are a few important things to
keep in mind when you are upgrading to
the final version:
The Windows 7 beta or RC releases were
Ultimate edition, so you’ll only be
able to upgrade to the RTM (final) if
you are installing Ultimate Edition.
Whenever possible, you should really
backup your files and do a clean
install. There are less headaches this
way, and you get the benefit of a nice
clean profile.

Note: I know your not asking for this answer, but the worst that can happen is that you loose some settings and have to reinstall some applications. However, everyone using the RC will have to do a clean install"