Dear mergen3107 et al,
Once again, many thanks for all those tips and ideas. I find it quite impressive and actually heart-wearming to encounter such a community of helpful and knowledgeable experts here, hence kudos to everyone!
Now with regard to the various questions and points:
1. The firmware version of the 'old' Kindle is 5.12.4
2. I have a total of about 300 books on that device. What I find a bit confusing that some of the folders mentioned are seemingly empty whereas have tons of different files. For a third group no folders exist whatsoever. All pretty confusing, I am afraid.
3. I am on a Mac and use calibre (
https://calibre-ebook.com/) to manage all those ebooks. Originally I had assumed the program would be able to handle the back and forth of all files including notes etc. but, alas, that does not seem to be the case.
Finally, now that I know that this should be doable and _think_ I know how to approach the problem, I have actually decided to wait for a bit before I start moving everything, until the new Oasis generation is out, to be precise. Once they are on the market, I will instantly get hold of one, start the migration process and surely report back should I run into difficulties (or not).
In the meantime, many thanks again to everybody, your help is truly appreciated, and keep up the good work here!
Very best
ACTG