200kHz is a common Freq however you should avoid taking the technical advise of sales folks unless you have some specific reason to think they they are qualified to speak on such matters. Ask the tech if he/she thinks the dual freq Furuno will work on the single freq old transducer.
Does not sound like a good thing to me. Hard to imagine that the dual freq Furuno will be happy with a single freq transducer, or that the plug and the number of conductors will even matchup. Further, the power output of the Furuno may exceed the design power of the 30 year old transducer. Think you need to bite the bullet and swap transducers. I have to do that on my next haul out.
Will warrenty cover the Furuno if the control head gets smoked by running on the old transducer?
Before the discussion goes much further you need to check the pinout / #conductors on the old vs the new unit, that may decide the question and end the discussion.