I know what you mean about “fixing” the pastels (the old ones). I have sometimes touched up a watercolor or an acrylic painting with pastels and then regretted it—not because it didn’t improve the painting but because the damn pastels slowly rubbed off. When I tried to“fix” them with a varnish spray (fixative) they changed color, or at least lost their “edge”, their brightness.
I read about Degas’ constant struggle with his pastels and his experimenting with fixatives. And I read somebody (or talked to them?) who worked in a museum that exhibited some Degas pastels. They are under glass for protection but “there is often fine pastel powder on the floor under the glass,” she said. Imagine! That means the great works are losing their edge too.