Just out of curiosity if nothing else I've created a plugin with a command that should swap the note type.
The command to run is toggleMarkupLanguageType and once the plugin is installed you can do it from the command palette.
I haven't published this yet as I'd like to know if this actually breaks anything before I unleash it on the world so currently the way to install it would be to download the jpl from my github repo Release v0.1.0 · Daeraxa/joplin-plugin-toggle-markup · GitHub and install it manually in Joplin, preferably in the dev environment as I accept no responsibility for breaking anything.
If you decide to install on your main environment then please make sure you back everything up first.
Code wise it doesn't really do anything scary (in theory). It just grabs the selected note's ID, checks its markup_language type then sets it to whatever it currently isn't.