Create your new imap account then hold and drag the old emails across to the new account. There will be a new imap folder. For a lot of emails this may take some time. You can of course leave your old emails on Outlook but then other devices would not see them.
If you are using the same email address you may hot some snags when creating the new imap account. To be on the safe side back up your old emails bu exporting to a pst file first. It is possible that you may have to delete the pop account.