In terms of exporting existing Day One entries I eventually settled on export as markdown, splitting (because wretched Day One insists on exporting all selected entries in a single, combined file) and importing into Day One. Although I mentioned the process on this forum I have since then posted a rather more refined and updated version on the Hazel forum.
I prefer having each individual entry as an individual markdown file (which enables creation of links between them and tagging individual entries if that’s something you want to do) to having one or two large pdf files. (I should say I have over 18,000 individual journal entries in Day One—now all imported and grouped by year in DT!)
Stephen