Nothing new here; it's all been said before, but I was documenting the steps for someone and figured adding a few more minutes would relieve me from doing it again. It's a bit different for now because the conversion relies on Thunderbird, which is in a state of major transition. Anyway, if anyone is considering switching to IMAP from POP, this page may help: https://davidkirk.org/thebatpoptoimap/
Smart move, Daniel. Normally, Thunderbird and its addons are working together, but the leap from version 68 to 78 left many addons playing catch up. And there is still version 60 that is being left to upgrade, but customizations with version 60 aren't compatible upwards and I did not want to reference version 60 for the conversion, although it is quite adequate to the issue. They're on the right track, experiencing a few bumps as is TB!. After things settle down for Thunderbird, I'll update the site. But saving the files as you did ensures you have all the necessary tools. As an aside, if you only want Thunderbird for the conversion process, there is a legal portable version that doesn't update your registry at https://portableapps.com/apps/internet/thunderbird_portable It's the same code, but has a small launcher program. That would allow you to just delete it when finished. david
P.S. I just updated the site to reflect availability of the portable version and also acknowledged George Salnik's comment that running POP and IMAP together is okay.
Edited: david kirk - 22 July 2020 18:16:58(additional information)