At TechEd Europe 2010, I had the privilege to work on the VS2010 Ultimate booth. It was great to meet so many folk, and find out how different people were using TFS and VS2010.
One recurring question was about upgrading, and the ways to do it.
Neno Loje delivered a great talk about administration of TFS, in which he covered a range of topics, including upgrading. I recommend watching the recording to check how you are doing. The recording is at http://www.msteched.com/2009/Europe/DEV311.
He recommended an approach that will import the previous version in to a new instance of TFS. The advantages of this is that you can move to a 64 bit application tier, and have the certainty that the new instance is up and running before importing the existing data.
Recommended Reading
The devil is in the detail, and it is well worth reading up on the upgrade process:
