Newer Keynote (AppStore) won't open some old Keynote (iWork) files

So this is something that I've seen others have run into, but I haven't seen any sign of anyone having a resolution or even having gotten as far into understanding the issue.

I have to deploy an upgrade to someone with a fairly old Mac, running an old MacOS. They resisted for a while because they didn't want to upgrade to the new version of Keynote. We put off dealing with it for too long and now are in a bit of a spot.

My problem is that I have several files that open fine in the old iWork Keynote, but the AppStore version just gives a generic error that makes it look like the file is corrupt. ("Keynote couldn't read the file." error.) These files all consistently work fine on any computer capable of running the older Keynote.

I was actually able to trace this down to being an apparently bug in the processing of old Keynote files where something has happened in the 'theme' settings for some slides that it doesn't like. If you remove the slides in question (which is a lot of work to determine) the file will open. Also if you change the theme on the sildes in question it will open.

So far the quickest fix for these files that won't open is to open them in the old Keynote, and reset the theme for all slides (it's too much time to figure out what slides are the problem). The biggest problem with this method is I have to keep a computer around running Mojave in order to do this.

I'm posting this here in the hopes that Apple will notice that this is an actual long-standing bug with Keynote and can potentially come up with a fix. Ideally Keynote should be able to deal with the theme data it doesn't like by discarding it, rather than just not loading the whole file.

Also, if there was a tool to be able to run the old Keynote to allow someone to be able to fix these files on a modern Mac that would be good. Or a tool that would sanitize old Keynote files for things that make the import function of the new Keynote fail.

At this point, I think I'll have to concoct some scripts to identify Keynote files with the issue and to reset the themes on them. I suspect that this will not be as easy as I imagine though.

If anyone else has any other great ideas for getting past this, please let me know!