I have come across this a number of time and there is no easy way to do this. Microsoft has hinted at a way coming up in the near future, but for now the answer is to backup all your data and close down your P or E plan; open a P or E plan and import the data back.
A. To switch plans, you need to cancel your account and then sign up for the plan you want. You cannot migrate between a Plan P account and a Plan E account. In order to move between the two plans you would need to cancel your account and then sign up for a different one.
Office 365 does not support converting or migrations in the following scenarios:
Office 365 (Plan E1, Plan E2, Plan E3, or Plan E4) to Office 365 (Plan 1)
Office 365 (Plan 1) to Office 365 (Plan E1, Plan E2, Plan E3, or Plan E4)
You need to sign up again and provision a new tenant that is based on the subscription plan that they want, then go through a manual migration of the data from one tenant to the other (Exchange .PST backups, SharePoint Online backups, etc.)
Thanks for visiting and reading my posts. I am always looking for more ideas. Please comment or email me with what you would like to see.
Office 365 MVP