I can't speak to Windows, but this is pretty much standard operating procedure for many Mac apps. If you download a new version of Reason, for example, the old files are automatically mapped to the new version. Doesn't mean you can't go back to the old version, as you found with Windows, but if there is a new major upgrade to an app, macOS does map the files to the new application version.

Think about it; there is a lot of benefit to this approach. Let's say you deleted the old versions of Finale and your OS didn't automatically map your existing Finale files to F27 and you thus couldn't open them without telling the OS to open all .mus and .musx files with the new version. I think that would be a worse situation.

. I have several older versions of Finale installed… The Finale installation process should ask before doing this.

It’s not a Finale issue — Windows or MacOS and each handles this differently.

but if there is a new major upgrade to an app, macOS does map the files to the new application version.

Only if there’s one version. If there are multiple versions of the app installed, one has to change it in Get Info. There are many threads on this issue over the last seven years including one last week along the lines of “I just installed 27 but the files keep opening in 26 (or 25 etc.), how do I…”

Bryan,

I don't quite understand. You want Finale 26, for example, to also be able to open musx files along with Finale 27.3? I don't think that is possible as the default type. Both can coexists but one or the other should be the preferred one.  It is simple to assign mus files to an older version of Finale that is capable of opening that type file.

There is a screen that isn't readily apparent at the beginning of the Finale 27.3 installation that asks if you want to keep older versions. I too was a skeptic at first but I have completely gone over to Finale 27.3. I had 26.3 and 2014.5 on the same computer at the same time but soon found 27.3 to be so well behaved and improved it made the other useless. The JW New Tools is worth it alone in my book.

And you can always open F26, and then open the files from within. You just can’t double-click the file to open it.

Thanks, everybody. It actually is part of the Finale install script on Windows rather than something Windows automatically does--how would Windows know that .musx is the extension the application uses? I have installed other things that ask. Nevertheless, I'm sure there's a way to get those associations back. When I try again I'll looks for the "keep old versions" screen that Mr. Biggs mentions (thanks very much for that!); perhaps that's Finale's oblique way of saying "do you want ..musx and .mus associated with Finale 27?