I am back. And the news are good 😁
Just as I hoped, we can make good use of XML to solve our problem.
Here is a quick step by step:
1) Select your project, and do Export XML...
I recommend that, for now, you choose version 1.9 - this makes the next steps a little simpler (*)
2) Open the XML file with a text editor. I prefer TextMate, which is free, but you can even use TextEdit - just so long as you keep the file as Plain Text.
3) Do a global find and replace:
Find: <marker
Replace with: <chapter-marker
4) Save the file
5) In FCP do File->Import->XML... and select the file you just edited.
You will be warned that it may replace items already in your library. You may choose "Keep Both", so you will have the before and after versions of your project. Open the new one, and there you are!
I tried this with a small project with three markers (one of which was already a chapter marker) and it worked beautifully.
In fact it took much more time to write this post than it did to export, edit and import back.