No! All is not lost. All attachments and screenshots (with the exception some stored inline, we’ve noticed) carried over in the upgrade. However something went wrong in that when I ran the first migration sweep to bring in the bulk of the data, I did so while the forum was running under a temporary URL. The import settings allowed for one to anticipate for a “live” URL, and while that setting kept cross-references intact once the new URL went live, it unfortunately did not do so for attachment links.
I should have realised something was wrong, in other words, when all of the screenshots were working fine the first time I imported, because that is when they should have all been broken.
So anyway, there is a piecemeal measure wherein an admin or moderator (and maybe even you can run it yourself if you have edit permissions on the post; it would be in the ... button beneath the post) can “rebuild the HTML” of a post, which immediately fixes all of the broken URLs within it. I just did that for the macOS theme thread, for example.
That would be highly impractical on a global level though, post by post! So the main goal is run a global search and replace on the server side of things, which I believe should be possible, as I’ve found instructions for handling scenarios like this—though in that case where one switches their forum to a new domain and wants to deal with thousands of internal links in posts.
In the meanwhile, if you encounter an older topic you were meaning to link a user to, and notice a crucial screenshot is broken and cannot fix it, flag it for a moderator to look at.