Any tips for removing ancient steam-radiator air-vents without breaking?

Hi all,

Like most every old threaded connection in the old house, wouldn't be surprised if the the air-vents break off rather than unscrewing cleanly.

Detail: Single pipe steam heat. Cast-iron radiators. Air-vent valves not touched in a half-century. Would like to change-out as (observed at the end of the last heating season) the majority either let air and steam out constantly, and a few others take forever to let air out to permit steam to come in.

Thanks.