Hi Robert,
Currently, it is unavailable to directly upgrade your P plan account to an E plan account. However, if you’re using Office 365 P plan, you can change the domain’s name server records at your domain registrar to point to the Office 365 primary and secondary name servers. Office 365 then hosts your DNS, which means that the required DNS records for your Office 365 services (Exchange Online, SharePoint Online, and Lync Online) are configured for you.
If you have already changed the name servers to Office 365, then you need not to add the SPF record manually.
Please refer to Verify your domain and change name servers at any DNS hosting provider and domain registrar
Thanks,
Allen Zhang