Safe to run migration on live/production server?

I have an old mbox formatted INBOX and folders that need to be converted to a Maildir. Can I run the dsync command when the server is live and Dovecot LMTP is delivering new mails to the same user?

Will it be able to preserve UIDs and not confuse any indexes?

I started the job with deliveries disabled so the conversion could complete, but it's taking too long. I want to know if I can re-enable mail deliveries and not cause problems.

I see it building a massive number of files in the user's Maildir/tmp folder, which I presume is normal.

By the way, what's the size multiplier for mbox to maildir for many small emails only a couple K each? I'm finding it's a lot more disk space for maildir in this case.