Seniority
750 is about the point CMDs are up to on the PAX fleet.
966 is the most senior Captain on the classic (1st Jan 2000) excluding those on extension and AR.
1410 is the most Junior JF/O.
1850 is the most Junior S/O.
So when the PAX CMDs reach 966 the Freighter Captains will start to become eligible to move to the PAX fleet, however as petitfromage said appointments to the passenger fleet are subject to vacancies being available and other requirements i.e. CX will not allow them to leave unless they can be replaced.