I ran across what I think may be a bug in dsync when trying to convert from mbox to Maildir format. My first attempt failed with a runaway dsync process spewing out thousands of identical lines of the form:
dsync(buus): Error: Duplicate mailbox GUID 00fe691ff60da85b090d0000c8119523 for mailboxes Archive/Personal and Work/VendorInfo - giving a new GUID 00fe691ff60da85b090d0000c8119523 to Archive/Personal
These two mail folders happen to be among a number of folders I've had for many years, but also haven't looked at in many years. I used to use UW-IMAP, but switched to dovecot about two years ago, and I'm sure I've never used dovecot to look at those folders. So I suspect the root cause of the above error message was that dovecot had never generated GUIDs for these and some other really old folders.
After killing dsync with SIGKILL, which couldn't be killed with SIGTERM, I restored my mbox format Mail folder from backup, restored the mail_location and then coerced dovecot to generate unique GUIDs for all my mail folders with this command:
doveadm mailbox status guid doveadm mailbox list
After the above command, I was able to convert my mail from mbox to Maildir format.
Maybe this will help someone else somewhere down the road.