Can you convert a propane furnace to oil? How much would it cost?

I imagine that you can buy enough parts to totally replace the burner and controls to make such a convert but I think it would be cost effective to just buy a new furnace. You'd be putting a lot of money into a furnace that still has an old exchanger and blower.

Many Mobile Home furnaces are designed to be converted from gas to oil. Most regular home furnaces are not.

If it isn't too old, it may be worth while. Check the manufacturer's catalog. The same furnace may have come as an oil furnace too. Pick up a new burner assembly, pump, and control board and be in business. Your local heating oil distributor may be of help too.