Type of work: Residential upgrade

Situation: Several of the rooms have the old style ceiling mount metal BX boxes with a metal support bar that spans between the joist. Does anyone know if they have any type of accepted weight rating? I.e. Are they acceptable for paddle fans. I know they will hold one, but are they acceptable to use in retrofit work? I'm just trying to avoid putting a hole in the ceiling to change it out to a ceiling box that is "officially" rated.

Thanks