So I seem to have bumbled my way to a "solution"....
I rename the existing AC22 Classification (Name, Version etc) to match the AC24 Classification then import the latter.
After some protest and yelling from Archicad and some nervous clicking from me, the AC24 seem to overwrite the AC22's Classifications and all seems to be well again.
Is this how its supposed to be done?
Andre Steynberg
Win 10 64-bit
AC27