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How do I upgrade or migrate from P1 plan to E3 plan?
Here is one way to do it: migrationwiz.zendesk.com/.../20358091-how-do-i-perform-an-office-365-to-office-365-migration. Essentially this copies the data from one source mailbox to a desitnation mailbox while preserving the domain name. Note that the steps are simpler if you do not need to keep the same domain name (I'm assumimg you do).