Just as a heads up, most of the Technet articles I found on doing a SBS 2003 to Windows Server 2012 R2 Essentials migration cover Server 2012, not 2012 R2 and they all mention installing the server in "Migration Mode" and grabbing the "Server Migration Prep Tool" from a folder on the DVD called supporttools.
THESE DON'T EXIST IN R2.
Server 2012 Essentials R2 is a separate product than Server 2012 Essentials, and the two, tho related, do not share some bits.
I found out the hard way by doing an install, then seeing the Migration Mode stuff, and restarting the install, only to find there was no migration mode option.
But, they do have a document that spells it all out for you, without using the missing Migration Mode install... Lots of threads with "I just installed Server 2012 R2 Essentials and I am also not seeing the migration install option anywhere." messages. Happily someone pointed them (and me) here:
Migrate from Previous Versions to Windows Server 2012 R2 Essentials or Windows Server Essentials Experience which can be found here: http://technet.microsoft.com/en-us/library/dn408633.aspx
Posted so I don't lose the link, and maybe people Googling will find it easier than I did. 🙂