The notes are markdown and in the FAQs he mentions the location of the local data cache. I imagine you can simply copy all your notes from the cache, clean up any incompatible metadata, and import them to your next note app.
Don't let paying for software stop you if you can afford it. This looks like quality software and we should support it, if able. If you're looking for free note software that's tied to a company, Firefox has a note taking app that syncs between phones/web browser (it's a bit buggy and logs out at times), and Google has Keep that also has apps and available on the web.
I'm all for supporting the things that are good. But at what point do we say "enough!"? All of these subscription services start to add up, going from free to 5 USD a month is a bigger jump than I'm willing to take. Why aren't there any services out there that offer 1 USD/EUR plans?
Don't let paying for software stop you if you can afford it. This looks like quality software and we should support it, if able. If you're looking for free note software that's tied to a company, Firefox has a note taking app that syncs between phones/web browser (it's a bit buggy and logs out at times), and Google has Keep that also has apps and available on the web.