I have gotten questions from customers asking about installing Exchange 2013 into their environment. A keep point to note about coexisting with prior versions of Exchange is documented on: http://technet.microsoft.com/en-us/library/aa996719(v=exchg.150).aspx
Exchange 2013 will support a N-2 coexistence scenario which means that Exchange will coexist with 2 versions back which are: Exchange 2010 and Exchange 2007. Older versions of Exchange (e.g. Exchange 2003) won’t be supported for coexistence. This basically means that if you’re on Exchange 2003 or older, then you’ll need to upgrade to Exchange 2007 or 2010 before upgrading to Exchange 2013. In the past, third party migration tools (e.g. Quest) has provided tools to upgrade directly from an older version of Exchange, but this means that you are relying up that 3rd party vendor for support.
However, if you have Exchange 2007 or Exchange 2010, please note the statement that you’ll need Exchange 2007 SP3 + rollup or Exchange 2010 SP3 to coexist with Exchange 2013. However the Exchange 2007 rollup and Exchange 2010 SP3 haven’t been released as of this date. So as of now, Exchange 2013 can only be installed into new environments without prior versions of Exchange until these components are released.
I got some questions from folks asking when are these updates slated to be released. Here is a blog article from Exchange product team that says the target is the first half of 2013. See