Code:
on *:TEXT:*:#CHANNEL {
  if (($level($nick) >= 50) && ($msgtags(subscriber).key == 0)) {
    /auser 0 $nick
  }
  elseif (($msgtags(subscriber).key = 1) && ($level($nick) < 50)) {
    /auser 50 $nick
  }
}

not tested.

Dont give a fish - teach to fish!