Hello and good afternoon! This is an excellent question and I'd be happy to provide some guidance. The best way to migrate data from an existing CPanel server is likely using Settings>Connectivity>Mailbox Migration as this will allow you to leverage IMAP to sync the entirety of the account mail data into the new user's account in SmarterMail. 

If you need an automated unattended solution I recommend checking out IMAPSync/IMAPCopy. These utilities allow you to essentially script an IMAP migration between the old and new server for all accounts simultaneously. 

Alternatively downloading the email data into an email client, then uploading it to the new SmarterMail account should also suffice. I hope this helps!

We've never had any good luck with the SM migration tools. Though, I will admit, we have not used it in v17. We always use imapsync and it works beautifully!

Thanks both for your advice

I'm just running the  Settings>Connectivity>Mailbox Migration and so far so good.

What should expect with regards to now delaing with the client's local devices - MAc and phone?

Should I anticipate simply changing the account mail server settings? 

Or may that result in all the emails downloading again and duplicating?
In which case I should back up and delete the account and start with a fresh account.