Downgrading or upgrading Windows Server 2008 R2 Datacenter, Enterprise or Standard Edition on an Exchange 2010 server

I ran into an interesting problem last week when I noticed that the new Exchange 2010 servers’ operating systems were all the Datacenter edition when they should have been Standard and the reason why this happened was because the template that was provided for the virtual machines was built with the Datacenter version.  The challenge I faced was that the servers were in production and getting downtime for mail wasn’t going to be easy and when I do get a window, I need to be sure that I fixed the operating systems as fast as possible.

The first idea I had was to simply install a new mailbox, cas and hub transport server and have them operate in parallel with the existing servers but I realized that this wasn’t going to work because the client only had 1 set of license keys for Exchange and when the mailbox server is in evaluation mode, it would only map 5 stores and in this environment, there was 7.  In addition to the mailbox store issue, the thought of building and migrating the services to temporary servers and then swinging it back to rebuilt servers simply wasn’t efficient enough.

The next idea I had was to simply mimic an operating system corruption and use the Setup /m:RecoverServer switch to reinstall the Exchange 2010 roles back onto the server.  I went through the TechNet article here: http://technet.microsoft.com/en-us/library/dd876880.aspx and figured it was going to work but before proceeding to schedule the window, I went ahead and posted a message on our Microsoft Partner forum to verify that it was going to work but ended up being disappointed with their answers.  I was initially told that it was possible when I used the “corrupted C drive” as an example but then was told that it wasn’t possible when I told them I just wanted to downgrade from Datacenter to Standard version of Windows Server 2008 R2.  The response I got from the Microsoft staff was to carefully ready the following:

• The server on which recovery is being performed must be running the same operating system as the lost server. For example, you can't recover a server that was running Exchange 2010 and Windows Server 2008 on a server running Windows Server 2008 R2, or vice versa.

My interpretation of the above was that we cannot replace an R2 operating system with R1 and vice versa which made sense because they are quite different but I was not convinced that cross versions would be a problem so I gave up on our Partner support and proceeded with cloning the virtual machines in the production environment and trying it out.  So if you’re looking for the answer to whether you can downgrade from Windows Server 2008 R2 Datacenter to either Enterprise or Standard for an Exchange 2010 server using the recover server option, the answer is YES you can.