You can install the professional version as standard. VS does not support updating on the fly - fill in a new license key => new version. But you cannot use two different versions of the same version of Visual Studio.
This is not possible in the same system.
- Standard Version VS 2008
- Professional Version VS 2008
It is possible in the same system.
- Standard Version VS 2005
- Professional Version VS 2008
If you have an MSDN version, install more.