I do not remember the particular order, having spend 3 days trying to resolve this. After installing M4, when I started KMail for the first time, it exhibited several crashes, no emails were shown, KMail was very slow (still is, after various akonadi migration resources consumed about 2 days worth of CPU time). Then some mesages started to appear. KMail seems to be (still) migrating emails from ~/Mail (where my KMail folders were for the last 10 years) to the akonadi database at ~/.local/share/akonadi/db_data/akonadi (which is now over a Gigabyte, but that is fine, I seem to have many emails). It looks like I have to hit F5 on each KMail folder to migrate it. I did that now for all of my KMail sub-folders (all 100 of them or so) I have spend about 3 days watching KMail locked or very slow while this is hapening. 
The bad thing is it is all extremely confusing. It seems every time I open KMail, it is re-migrating some folders, showing about 50 “Filtering Message” in the popup in the right bottom corner. KMail is close to unusable.
The good thing is that it appears to have migrated (some?) of my emails to ~/.local/share/akonadi/db_data/akonadi.
This is where my question comes:
- When (and how) can I get rid of my old mail folders in ~/Mail, and make KMail start using only Akonadi database? (It appears the fact the old folder are still there, causes the constant synchronization attempts.)
Sorry this is not very clear, but I am not very clear on what is happening…
Thaks for any pointers on my question…