You cannot actually call it a migration, because the Outlook Add-in is identical for Jira Cloud and Jira Server/Data Center. The only steps you need to take are:
Install the related Jira Cloud App of “Outlook Connector for Jira”
Change the connection settings in the Outook Client. Or add a new connection, then you can use Jira Data Center and Jira Cloud at the same time.
That's it, there's nothing more to do. However, you can check whether the latest version of the add-in is already installed in Outlook to make sure everything is up to date for Jira Cloud.