Install an older Exchange version after last server was removed

Consider a scenario where an Exchange environment was upgraded from let’s say Exchange 2007 to Exchange 2010 (I know, old versions). Some time after the admin removed the last Exchange 2007 server they find out that they a LOB application is not working correctly and they need to add a server with Exchange 2007 to allow it working again. The question is: can we add an older Exchange version after we removed the last server from the environment?

A community member on Reddit’s ExchangeServer community recently stated that this would not be possible:

…you won't be able to install a new 2007 server into a forest with 2010 servers once you've removed the last 2007 server…

In a follow-up discussion the poster referred to this article as his source:

Question: Is it possible to install Exchange 2003 or 2007 in a pure Exchange 2010 organization?
Answer: [...] If you have transitioned from Exchange 2007 to Exchange 2010 and the last Exchange 2007 server has already been decommissioned, the answer is again no. You will not be able to install Exchange 2007 at a later time in this organization because it’s now considered a pure Exchange 2010 organization.

This excerpt is from an article in the TechNet Magazine and was written by a well-known Exchange export so I don’t blame the Reddit poster at all. However, with my knowledge of how Exchange works I could not think of any reason why this shouldn’t work and this is the first time I heard of it.

My position on this subject is that there is no reason why you should not be able to add an Exchange 2007 server in a Server 2010 forest after the last Exchange 2007 was removed.

From a technical perspective there’s nothing special happening after you remove the last Exchange 2007 server. The Exchange environment does not keep count of the number of Exchange 2007 servers and there’s no process that implements a blocker when the count reaches <1. Also there’s no such thing as a ‘pure Exchange 2010 organization’, unless you want to say that there are no other Exchange versions than 2010 in the environment.

Time to validate this in a lab:

1. Create a domain controller for a new forest, DFL and FFL set to Windows Server 2003
2. Add a server with Exchange 2007 SP3
3. Add a server with Exchange 2010 SP3
4. Move the mailboxes and OAB generation to Exchange 2010
5. Remove the Exchange 2007 mailbox database and storage group
6. Uninstall Exchange 2007
7. Add a server with Exchange 2007

As expected I was able to add the Exchange 2007 server to the environment without any issues.

Now in a real world environment there can be other factors involved but’s it’s good to understand that your Exchange organization does not add a block when you remove the last server of a certain Exchange version.