Team Foundation Server 2010 is not compatible with Visual Studio / Team Explorer 2005 and 2008 without the compatibility upgrade.
In order to install the compatibility pack for Visual Studio 2008 go here:
In order to install the compatibility for Visual Studio 2005 go here:
Once you have connected to the new TFS server and mapped your projects back, you should be able to map your workspaces without any problems.
However, you might find that your workspaces do not work. This is especially an issue in Visual Studio 2005, which seems not to recognise the workspace that you have reapped the workspace. If this is the case, run the following command from the VS2005 Command Prompt:
tf.exe workspaces /remove:*
Then get latest on your source branch. It should be OK.