I am currently in process of upgrading out exch2007 to exch2013. I have all clients connecting to my 2013 CAS server. 2013/owa users can login with no problem and access their mailbox. If the mailbox is on exch2007 it does not redirect to legacy to locate the mailbox (From internal and External).
Ive even manually added host Files on local machine for testing as follow:
*Virtual Directories are setup in 2013 and legacy in 2007.
2ndly the outlook cleint of a 2007 Mailbox keeps asking for authentication and never connects.