I think they are changing the old warheads with hypersonic ones on the old missiles.
They didn’t use kheybar shekan missiles, wich had been very successful in previous attack, and we saw that missiles in Yemen that looked like a kheybar shekan with a Fatah warhead, so I think they refitted those first, since they are the newer models.
Those were apparently detachable boosters of the marv. The warheads are really cheap when compared to rocket fuel.