[Note: this is a shortcut variation on the steps in the Technet article on http://technet.microsoft.com/en-us/library/jj134039.aspx and should for now only be used in lab scenarios as it hasn't been officially tested by the PG's]
The short version;
- Add the AD FS role on Windows Server 2012, choose to add it to an existing farm.
- Make the new ADFS 2.1 server the primary (set-adfssyncproperties -role PrimaryComputer).
- Make the original ADFS server secondary (set-adfssyncproperties -role SecondaryComputer -PrimaryComputerName <computername>).
- Update DNS to Point to the new server
- Remove the original ADFS server
The long version; http://technet.microsoft.com/en-us/library/jj134039.aspx