216 was supposed to be the third UK Bucc squadron until the Red Flag accident revealed the extent of the wing problem, and following repairs there were only enough aircraft to form two squadrons plus the OCU.
Can't recall if the Seniority rules wree in force then, but it was a historic number so the 'change of role' was to resurrect it.
As to why 12(B) was resurrected, the planning and formation of II onto the Typhoon was in motion long before the decision to retain a Tornado squadron, so it made sense to continue that and temporarily reform 12. saves another re-shuffle in 12 months