The way things work with Sherlock Holmes, some of the original novels are in the public domain (because they were written before the cutoff) and and others are not (because they were written after the cutoff.)

Sherlock himself is in the public domain. And he plays the violin. That’s also in the public domain. But he didn’t get a dog until a later book, so that’s still under copyright. And he didn’t get a diagnosis of bipolar/manic depression until later, so that’s also under copyright.

So if you want to write a book about Sherlock, that’s fine just as long as you don’t give him a dog or bipolar or otherwise use any elements from after the cutoff.

So, I believe the original trilogy will be in the public domain in 2073 and later additions (Han shooting second, musical numbers added to the special edition, etc) won’t be in the public domain until later.

If the “Despecialized Edition” was made from the 4K version, I think it’s certain that the elements added after the original trilogy’s release would be remain under copyright for a time after 2073. But if there are any scenes that are straight up one-to-one identical between the original release and the 4K edition, I’d guess whether those would be under copyright after 2073 would depend on whether restoration and/or upscaling qualifies as sufficiently “creative” to warrant it being a distinct copyrightable work from the original release version.

Now, even just the act of making copies of parts/all of either the original or 4K endition (in the process of making the Despecialized Edition) probably qualifes as copyright infringement. So those folks definitely would be in trouble if Disney went after them. But after 2073, whether trading copies of the Despecialized Edition will be illegal will depend on whether upscaling and restoring qualifies it for fresh copyright, and also whether any copyrightable creative elements from after the original release are included in the Despecialized Edition.

Sorry. That’s a lot of words just to say “I dunno.” But I think it’s fair to say that after 2073 if someone upscaled the original theatrical version from that DVD release that had it (assuming nothing new was added to that not in the original theatrical release), it’d be perfectly legal to create and trade copies of.