If you can remember back when you built you Exchange environment, you ran setup /PrepareAD and you may or not have noticed this message:
Setup will prepare the organization for Exchange 2013 by using 'Setup /PrepareAD'. No Exchange 2010 server roles have been detected in this topology. After this operation, you will not be able to install any Exchange 2010 servers. For more information, visit: http://technet.microsoft.com/library(EXCHG.150)/ms.exch.setupreadiness.NoE14ServerWarning.aspx
Now if you forgot about this, your are screwed. In this case I would not be able to install any Exchange 2010 in the future.
Wouldn’t it be nice if Microsoft made you press any key or something, so if you have a DOH! moment you can fix things?!