Hi Scott,
Yes. If only one mailbox failed to migrated, we suggest you delete the old batch and create a new one only for this mailbox. Or you can use pst files to migrate this mailbox manually.
To modify the initial batch will cause some unexpected issues such as duplicates.
In addition, until you change your MX record, email sent to users is still routed to their mailboxes in the IMAP messaging system. After a mailbox is successfully migrated, the mailbox on the IMAP server and the Exchange Online mailbox are synchronized once every 24 hours, in a process called incremental synchronization, until you stop or delete a migration batch.
Thanks,
Kid Yang