Earlier quoted context omitted.
I have hypothetically done the same thing, but it didn't require switching reader software, which is why I didn't understand how the switch to KOReader was connected to that... If all you want to do is read EPUBs you purchased from somewhere else, there's easier ways than jailbreaking and switching to KOReader.
I don’t really like having to convert EPUBs to mobi. I have had issues with Calibre breaking formatting back when I used a Kindle DX in like 2011. I am sure it’s better at conversion now, but it left a bad taste in my mouth, so I like being able to immediately drop the EPUBs into a custom folder and point KOReader. Additionally, KOReader is pretty nice in its own right, like being able to use any arbitrary TTF font a…
I use Amazon's conversion tooling rather than Calibre (first kindlegen, now amazon.com/sendtokindle), and haven't noticed any formatting issues - and the website gives me wireless sync support too. No arbitrary TTF support or custom screen images, though.