There's an option in Thunderbird called
Edit as new... which converts a message to a draft.
I am looking for something similar in Exchange.
Reasoning: We migrated old e-mail of a user from an IMAP server to Exchange. The user had many drafts saved. According to exchange though, the messages are not drafts anymore. My hunch is that, if we find a similar procedure as Thunderbird's
Edit as new..., that should do the trick.