Buccaneer S mk 1 conversion to S mk 2

Having read extensively on the subject I am unable to ascertain why there was never a program to re-engine the Navy’s (at the time almost new) Buccaneer S mk 1s to S mk2 standard.

At least one pre production aircraft (XK526) completed as an S mk1 was converted to S mk2 standard. The only structural change of any significance being the larger intakes necessary for the greater mass airflow.

On the other hand, very many Sea Vixen FAW 1s were converted to FAW 2 standard. This involved what would appear to be a much greater amount of work.

I would be very grateful if anyone can answer this question.