Just like the girls and them...once you go BIG you can't go small again.
Seriously though, a Big Maxi Rights cannot go to a 15 seater or less.
It can however go to a Joylong (18 seater)
There was a clause back in the day that allowed you to go from Big to small, but it required medical officials to confirm that a big maxi would be to strenuous to operate, but I think now they just declare you unfit to drive altogether.