Exchange 2013 is now available to the general public. A 180-day trial version can be downloaded here:
http://technet.microsoft.com/de-de/evalcenter/hh973395.aspx
Information on the system requirements can be found here:
http://technet.microsoft.com/library/aa996719(EXCHG.150)
Exchange 2010 and Exchange 2007 cannot yet be migrated to Exchange 2013, as Service Pack 3 for Exchange 2010 and the corresponding rollup for Exchange 2007 are not yet available.
SP3 is planned for the first quarter of 2013.