I THINK the local_mbox stuff is the old version that can be deleted. You might try renaming the directory and see if it causes a problem. If not, then delete it. Also, check the dates on the files. If there are none after the time you upgraded, you should be OK deleting the directory. I wish I could find something in the help to verify this, but I haven't yet.
Since upgrading, I've also encountered another problem. When I send mail, I was getting an error message:
I had to go into my mail accounts and set the default directories for Sent and Draft messages.
The reported error was "Failed to append to mbox:///home/bilkay/.local/share/evolution/mail/local#Sent: Invalid folder URI 'mbox:///home/bilkay/.local/share/evolution/mail/local#Sent'
Appending to local 'Sent' folder instead.".