The environment I administrate include the following servers at the moment:
2x Exchange 2010 CAS Server (as an CAS array)
2x Exchange 2010 HUB Server
2x Exchange 2010 Mailbox Server (as an DAG)
Users use MS Office 2010 (and since today Office 2013) to access Exchange from internal via direct connection to the CAS array and from external via Outlook Anywhere via a TMG 2010.
The question is now, could I reinstall ONLY the Exchange CAS Server with Windows 2012 and Exchange 2013? Will the Exchange 2013 CAS still be able to proxy to my "old" Exchange 2010 servers? I read in a blog that this should be possible, but I´m not really sure if that is a way Microsoft would support.
Note:
The next point would here to remote the DAG, reinstall one from the Mailboxservers with Windows 2012 and Exchange 2013 and move the mailboxes from the 2010 to the 2013 Exchange Server.