Hi Tmh,
As far as I know, to change from Office 365 P plan to E plan, you need to subscribe to a new Office 365 E Plan, and then manually move the SharePoint Online items.
However, when signing up for the new subscription (E Plan), it is not feasible to use the old tenant domain name (example.onmicrosoft.com), because this domain name is already used by your old subscription. You can try a new tenant domain name.
Thanks,
Young Yang