Will,
Its money. I imagine most Thales operators have just upgraded any existing AA to AB probes. I am sure that No 1 and No 3 will be changed and it will be a good test bed, to see if the F/O's air data goes haywire, to see if the Capt's and Stby are operating correctly.
It will also be about supply. If the figures are correct, then 400 new probes are going to be needed. I don't know if the ADM's are generic or not. If not, then 400 of them too.