Is there a way to transfer a Lightroom catalog into a Lightroom Classic catalog?

There might be a way.

You can try enabling sync from a new catalog. The following alert will appear:

Unfortunately the dialog box design did not anticipate my long catalog names, so it cut off the end of the message. The rest of the words are “…this catalog.” The “Click here for more information” link goes to this web page, which you probably want to review:

Sync a different desktop catalog with Lightroom for mobile

On that page is a note that seems critical for what you’re trying to do:

Note: Since Lightroom syncs only Smart Previews to Lightroom for mobile, for those images that were synced from Lightroom desktop app, Smart previews are downloaded and Originals are mapped if located at the same location from where the photo was first synced.

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Notice that last line: “…originals are mapped if located at the same location from where the photo was first synced.” If that means what we hope it does, it might be the relinking that you want, if your originals are still in exactly the same folder hierarchy that they were in when the old Mac died. But I’m not sure because I have never gone through this. (I clicked Cancel when I made the alert above appear.)

A caution is that document is old enough that it was written before Adobe switched to the Lightroom Classic name. And so the way the note is written is somewhat ambiguous, because it’s talking about “images synced from the Lightroom desktop app.” The context suggests that it really means the Lightroom software we know today as Lightroom Classic, not today’s cloud-based Lightroom desktop app.

To guard against unanticipated sync issues like replacing or removing files I wasn’t expecting. I would only go ahead with this after making complete backups of all volumes that need to be mounted during this process.