Thermodynamics aside speaking from exp of one or two injector changes, I have to change them in sets of 12 the heat and re-use method is only EUO. Heating and slowly cooling
is annealing heating and quenching is tempering. Advising someone to use these methods outside of a emergency
situation you are assuming a lot of things will go right. They will not over or under heat the copper and all scale will be polished or planished off to ensure a good seat. At the cost of pennies per washer it is better to replace. It should also be noted that cleaning
out the seat when you do injectors is of the utmost importance. Ideally you can use a spare injector to make a wood or plastic piece that is the shape of your nozzle fitted to a rod with enough tol for a tight fit with a rag over the whole works. You can use a clean cotton rag around this to go over your seats when you pull the injectors and ensure they are clean for the new ones. It only takes one little piece of crudd to make the diff.