[rashkae]

I thought I would update it with some information I posted in another thread. Several people commented how Epub converted to AZW3 retains all formatting. I think people who have been doing this a long time forget how much Calibre default settings changes things. Calibre default conversion settings are well optimized for converting non-ebook formats into E-books, (word documents, web pages/html, rtf documents, etc.)

However, when converting E-book format to E-book format (epub to AZW3, or vice versa), here are some changes to Calibre defaults that will preserve more of the original formatting.

In Look and Feel, check "Disable Font size Rescalling"
In Page Setup tab, change Output Profile to "Tablet"
In Structure Detection Tab: Change Chapter Mark to "None"
Change Insert Page Breaks Before to "/", (without the quotes.)
Heuristic Processing Tab, unchech "Enable Heuristic Processing"

The one big caveat: some books specify their fontsizes with generic keywoards (small, medium, large.)... With font scaling disabled, Calibre will mistreat those badly, (changing them to fixed point sizes, very bad.)... you will usually know this if previewing the converted books and the font sizes have all become tiny. In that case, you will either have to enable font size re-scalling, or change those font sizes in the source epub manually.
In the case of enabling Font-size scaling, I prefer to use this font-size key,, It reduces the shrinkage of smaller text: Base font size "12 pt" Font Size Key: "10.5, 11.0, 11.5, 12.0, 13.5, 17.0, 20.0, 22.0, 24.0"

Edit: Personally, in my experience, I find the Kindle, with the help of Calibre to side load, makes a better epub reader than the Kobo.