Thanks Adriano! I had completely overlooked the import/export function in the Types menu.
I tried your instructions, but when I tried to import the xml back into the site, I got an "invalid data" error.
I've tried to brainstorm possible causes, and have come up with three:
1) Problem with the format/content of the zip file itself
I really don't thin this is the cause of the problem, but I'll put it up here anyway. I used macOS's built in "compress" function to re-zip the files after saving the changes I made to settings.xml using xcode. I kept the same name for the zip file as the original export from Types. The new zip file contains settings.xl and settings.php
2) Problem with the data inside the xml file
Here's what the original data format was:
<wpcf-fields-select-option-cce7b4790485622655069c933ed2a0c9-1>
<title>Armenia</title>
<value>12</value>
</wpcf-fields-select-option-cce7b4790485622655069c933ed2a0c9-1>
<wpcf-fields-select-option-efaeb11ed82431ca12bb728a46cbccd6-1>
<title>Aruba</title>
<value>13</value>
</wpcf-fields-select-option-efaeb11ed82431ca12bb728a46cbccd6-1>
<wpcf-fields-select-option-3291479ee6bfd7baabf376d85c99d73b-1>
<title>Australia</title>
<value>14</value>
</wpcf-fields-select-option-3291479ee6bfd7baabf376d85c99d73b-1>
and here's what it looked like after I replaced it with 268 countries (only showing three here):
<wpcf-fields-select-option-008ARMf8028fb57d3ac131f2afb135ce-1><title>Armenia</title><value>8</value></wpcf-fields-select-option-008ARMf8028fb57d3ac131f2afb135ce-1>
<wpcf-fields-select-option-009AUSf8028fb57d3ac131f2afb135ce-1><title>Australia</title><value>9</value></wpcf-fields-select-option-009AUSf8028fb57d3ac131f2afb135ce-1>
<wpcf-fields-select-option-010AUTf8028fb57d3ac131f2afb135ce-1><title>Austria</title><value>10</value></wpcf-fields-select-option-010AUTf8028fb57d3ac131f2afb135ce-1>
I can't see any problems with this. Can you? I didn't want to break it down into the proper lines because that would take forever. That shouldn't a problem, right? (apart from being difficult to read)
3) Problem with characters (eg, UTF-8 etc) inside the xml file because I did a lot of copy and pasting between excel, sublime, and xcode