you can not use a VL Key for an oem installation an vice versa.
>Can I just contact Microsoft at that point and get a different activation code?
you should try this, because it`s a microsoft/windows issue, not a vmware issue
you will not be able to change to the Volume license key, however you should be able to use your original license key and re-activate it with MS. (not entirely legal as the OEM license is tied to hardware), the only other way I can see is that you P2V the server, then "Upgrade" your OS to the Volume license version once migrated. you will then be able to utilise your Volume license key.
I recently went through this. Here is what worked for me:
Go through the P2V process and when you start up the VM for the first time follow the following steps to perform an in-place upgrade of Windows Server 2003:
1. Insert the VLA Windows Server 2003 CD in the CD-ROM drive or the DVD-ROM drive, and then restart the computer. Windows Setup begins.
2. Press ENTER (to continue) when the Welcome to Setup window appears.
3. Press F8 to accept the License Agreement.
4. Use the arrow keys to highlight the Windows Server 2003 installation that you want to upgrade, and then press R (to Repair).
5. Windows Server 2003 Setup performs an in-place upgrade of the existing installation. Follow the instructions on the screen to complete the process.
The above information was from http://support.microsoft.com/kb/816579/