technically - the ATTRIB command is what is sent by the PCD to indicate that DR should be changed, and the Answer to ATTRIB (from the PICC, at the starting DR) is what is telling the PCD that the card understands this DR change command - then all communication after this would be expected to be happening at the new DR.
there are very few tags out there which are running data rates higher than 106kbps, though - and i see that we don't spec out operation as an emulated card above 106kbps in the -70A data sheet - i will ask the design guys about this and repost.
would it be an option for you to use P2P here between the two TRF7970As? This known to me to be working for NFC-A and NFC-F at 106kbps, 212kbps and 424kbps..