As an interim migration step, that will last at least for a year, I have an Exchange 2010 server co-existing with an Exchange 2013. Both fully updated with latest Windows and Exchange updates. After moving the DNS namespace to the new Exchange 2013 CAS I was doing some tests with a test mailbox named "xch2013setup" placed on the xch2013 Mailbox DB. I still haven't moved the arbitration mailboxes to Exchange 2013.
Mailflow (internal and external) is ok. Proxying between Exchange servers is ok. Opening Outlook client or OWA, from a Windows notebook, with the xch2013setup account, correctly connects to 2013 CAS. OWA from the notebook loads the OWA 2013 Premium UI in desktop mode. Connecting to a mailbox residing on Exchange 2010, correctly loads the meant mailbox, and, regarding OWA, I'm presented with the OWA 2013 login page and, once logged in, I'm proxied to Exchange 2010 OWA. Then, all seems to work flawlessly.
However, if I try to open the same mailbox from, for example, an iPad or an Android smartphone (tried with some newer and older OS versions), I'm presented with the OWA Premium UI for mouse layout with IOS and the Light layout version (html4) with Android. If, from the same devices, I enter in the browser address bar "https:///owa/?Layout=tWide" (or tNarrow) it loads the correct Premium UI touch optimized layout.
I know Exchange 2013 is in extended support phase and maybe I cannot pretend too much (even if a supported solution should work as expected I think, like it is proudly said here), but I wanted to know if the behavior is related to the 2013 server side OWA app that is aging, if it's a coexistence problem, or just a matter of an OWA virtual directory setting...