Exactly.I have never seen a manufacturer use hot melt to build screw ons.You can get them loose but it takes more heat for a longer time and that's plenty to destroy temper.Blades on vented heads get hot faster too.
There is Heat Stop paste and other similar products that will protect part of a metal object while you heat the rest of it but a tub of it will cost app. $16 and you will have to take the time to coat each blade well,while leaving the ferrule clear.
You may be as well off buying new heads.