As Preview Version has been replaced with the Exchange 2013 RTM version from the Download Center
You can Download from the Below Link
http://technet.microsoft.com/en-US/evalcenter/hh973395
Where Still Exchange 2013 RTM doesn’t Support Co-Existence
Looking for Coexistence – Wait Until Exchange 2010 Service pack 3 Releases
Download and Explore the RTM Version
Good Luck 🙂
Refer below link for Key Architecture Changes :