In one topic you mention the 6 cards that would be replaced if Seaside got upgraded. Have you made lists like that for other expansions?
No. Well, there's a post from a few years back where I go over things I'd change about expansions, but it didn't specifically consider the update pack scenario, and so was card changes that I can't actually make. When doing the new cards for the main set and Intrigue, it was a question, what about Seaside, and I thought a little about what I'd do. Prosperity could also improve, but Seaside needed it more and couldn't get it, so I didn't worry about Prosperity.
Alchemy just wouldn't be worth the effort; I could spend months getting a nice large set out of it - changing too much to have an update pack - that some people beg for but which many would automatically have no interest in. It is trivial to find a better project than this.
For Hinterlands the changes I want are trouble, they mostly involve making cards slightly worse, and I can't just make the simplified/weaker versions because the update pack that would have to exist would be a truly awful product. I don't want to replace the cards with new ones but that's the best I could do.
And then, Mixed Box and Dark Ages don't have so much to fix. Well overpay is too wordy of a mechanic but uh replacing it completely doesn't sound like a good project when whatever it was could go into a new expansion instead.
One thing I didn't consider at the time that might have been possible is a mixed update pack - seven new cards that were spread around between expansions. I haven't put in any work there either, I just see now that it might have been an option.