I agree. It IS possible to migrate fluently. However, do not forget that your 12.0 config master cannot be pushed as is on a 12.5 setup. In our SMA, we created 2 branches, one for 12.0 appliances and one for 12.5 appliances. We did a quick migration because it was complicated to maintain both 12.0 and 12.5 config masters at the same time.
Oh i was in impression SMA was already upgrade to 13.X to meet the 12.X WSA requirement, so you need to Upgrade SMA first to higer Level before you upgrade WSA, make sure you take the config back WSA / SMA out of the box.