[Dovecot] dsync mirror to convert from mbox (1.2x) to mdbox (2.2)

I'm converting mail from mbox to mdbox. I've copied mail over to a new system, and now I'm running dsync like this:

dsync -u username mirror mbox:/folder_location/username:INBOX=/inbox_location/username

That seemed to work, with a couple minor errors:

dsync(username): Warning: Our dotlock file /mail_location/username.lock was deleted (locked 20 secs ago, touched 20 secs ago) dsync(username): Error: file_dotlock_delete() failed with mbox file /mail_location/username: No such file or directory

Do I also need to specify where dovecot's indexes are for the old mbox files, or is this not needed for mbox conversions (when they are the source)?

Thanks, Ken A.