IMHE, they flatten by getting larger in outside diameter and smaller
in inside diameter..... they can be a pill to get off then...
depending on how far they have ridden up on the radius between the
threaded portion and the flange.

The thousands of copper gaskets I have changed have never shrunken
in ID and stuck on the plug. Automobile plug gaskets will, but they're
a hollow gasket that will reduce in ID when compressed. The coppers get
chucked when they get too big on the ID, as all of them eventually do.
I check every gasket before using it, as the annealing process will
blister it if it gets too hot. I use a propane spitfire torch; gets
really warm really quick. Annealing 100 gaskets takes maybe ten
minutes.

Dan