you must not lose the dbox index files, as they can’t be
	regenerated without data loss.

I have read this also, and was also worried about this, but when I look at the flat m.988 file, I still have quite a lot of useful data there.

"Note that with dbox the Index files contain significant data which is held nowhere else. Index files for both sdbox and mdbox contain message flags and keywords. For mdbox, the index file also contains the map_uids which link (via the “map index”) to the actual message data. This data cannot be automatically recreated, so it is important that Index files are treated with the same care as message data files."

https://doc.dovecot.org/admin_manual/mailbox_formats/dbox/