https://en.wikipedia.org/wiki/Apple_Studio_Display#Technical... has a good table. The short story is the 2nd generation Studio Display has some minor noted user facing changes but isn't that big of a difference. The Studio Display XDR is a bit of a merge of that and the old Pro level feature set.

Few enough differences so that if I could get an old Studio Display at a discount, I would. But right now it seems the old one is still full price where it's available.