Performing a migration to a 2012 FL Active Directory domain/forest?
Microsoft has published a blog post somewhere that I can't find at the moment stating that a new version of ADMT will be released in Q1 of 2014 that will support installation on, and migration to/from, any currently supported version of Windows Server. So, you have three options:
Shell out cash for Quest tools which don't have this restriction
Install a 2008 R2 DC in your 2012 environment. Lowering the DFL/FFL isn't terribly difficult and the only features you really lose are Kerberos armoring and CAP for file services. Note that you'll also need a 2008 R2 member server in the target domain to install ADMT on, unless you install it on a DC which generally isn't recommended.
Sit tight and wait for the next version of ADMT to drop.