Hi Mattcpi,
Currently, it's not available to change P plan of Office 365 to E plan of Office 365 immediately. If you want to switch from P1 plan to E1 plan, you need to cancel the P1 plan and subscribe a new E1 plan tenant. And as far as I know, I cannot confirm that this feature will be available on next version of Office 365.
Thanks,
Bourne Zhang