Unfortuantely the two accounts you have are significantly different and you will experience considerable difficulty attempting to simply change the mail settings.
We also do not know whether your current email is Exchagne-based (but configured for IMAP) or if it's, say, an alterntive technology (such as any Linux mail server).
The best approach will be to export your existing mailbox to a PST (ensuring you check each option to ensure mailbox, calendar, contacts are contained in the export), create a new Outlook profile and configure it to work with Office 365 and then you can import the existing IMAP PST file into your new Office 365 profile.
I have done a number of such scenarios and the quickest approach is what I describe. It also mitigates data loss as the existing data is left intact.
Thanks Kinnectus, only problem is I can't create a new profile as it has the exact same email address and outlook complains that it already exists - therefore I'd have to delete the profile to create the new one? – John – 2019-07-18T10:18:04.297
1@John You seems to add email to one profile. To add a new profile, go to Control Panel > Mail > Show Profiles > Add – Perry – 2019-07-19T08:42:05.850
@Perry thanks so much, forgot about profiles! – John – 2019-07-25T20:19:15.707