As already announced the CU1 for Exchange 2013 was released today. The update now also supports coexistence between Exchange 2013 and Exchange 2007/2010.
Details about the CU1 can be found here:
The Technet articles have not yet been updated. The CU1 contains the complete Exchange setup package. This means that existing Exchange 2013 servers can be updated with the CU1, and new servers can also be installed. The update can be downloaded here:
http://www.microsoft.com/de-de/download/details.aspx?id=38176
However, the download page seems to be a bit overloaded at the moment, my download is bobbing along at 120KB/s...
@Michael: If you're reading this, it's probably not going to happen today with your changeover 
If my somewhat stressed schedule allows it, I will start creating a first HowTo for the migration tomorrow. This should be available in the next few days.
Hi Frank,
tja aber morgen ist auch noch ein tag.
Aber die Vorbereitungen sind soweit durch. Mein Download ging wenigsten mit 2,3 MBit/s durch.
Grüße Michael