The duct work for my A/C and heat is sheet metal and 40 years old.It has started sweating on the ceiling.I have been told to replace it with new duct work.Is this a good idea or not?

 

Is it insulated at all?
Old duct sometimes was only designed for heating and not cooling. Did you recently have air conditioning added?

 

What's changed? Why did it suddenly start sweating? Did someone lower the cooling setpoint to cool the home quicker and left it so the AC ran all the time and overcooled the home and ductwork or what (I spec R4 on all supply ducts in the "Conditioned Space" to dumbproof it for that kind of thing)?

There's no need to replace the old metal ductwork, BUT you should clean it and upgrade it to today's sealing/insulation Standards. Might be less costly to replace with metal complying with current Code, not sure. Do not go for fiberglass ductwork whatever you do: I've never seen any that complies with the NAIMA RESIDENTIAL Standard, and metal ducts cost about the same as compliant fiberglass ducts, without the worry about fiber shed.