When you install VS from your MSDn subscription you will note that the
product key/activation code is already filled in and grayed out. I've never
had to "fill it in" as it is preloaded for you.

OK

So I have now found the Scott Guthrie blog (November 2007):

--------------------
Title:
Answers to a few VS 2008 Trial Edition, TFS 2008, and SourceSafe Questions

Text:
Below are some answers to a couple of questions about the recent VS 2008
release that I've seen the last few days:

[...]
Question:  Can you convert a trial edition installation of VS 2008 to a
purchased edition, or do you need to reinstall it?

Commercial versions of VS 2008 ship with a PID product key.  You can upgrade
a VS 2008 trial edition install to use this PID without needing to reinstall
VS 2008.  You can do this in one of two ways:

1) Pull up VS Setup from the add/remove programs tool in Windows.  Enter the
PID of the license you purchased and your upgrade is complete (no DVD or
separate setup required)...

----------------

It looks like I can't do this, doesn't it?

If this is a managed newsgroup, I understand that I should get feedback from
MS within 24 hours.  Well, I haven't.  And I now have one day left on the
clock for my trial version and a dead line looming...

Andrew

The answer to this problem is as follows:

1.  If you installed Visual Studio 2008 Team Edition trial version and then...
2.  Purchased an msdn professional subscription...
3.  When your team edition trial version expires, you have to:
3.a Download Visual Studio Professional from the membership benefits website
for your msdn subscription.
3.b  Extract the installation files to a folder on your pc using IsoBuster
or similar.
3.c  Run the installation from the folder, UNTIL you see the greyed out
product key.
3.d Copy the greyed out product key onto a bit of paper.
3.e Cancel the installation.
3.f Go to the Control Panel -> Add/Remove Programs
3.g Choose Visual Studio Team Edition -> Change/Remove.
3.h Choose Repair / Reinstall
3.i  Use the product key you copied onto paper in 3.d
3.j  The repair installation will detect the existing installation, lock
down the team edition features and leave you with a working edition of Visual
Studio 2008 Professional Edition, which is what you own and want.

Is this not a managed newsgroup?  Was I not entitled to feedback from MS
within 24 hours?

Grump, grump.