Hi Christian,
I’ll answer your questions separately:
Q1: How do we perform a migration from P1->E3? Having only recently completed a migration from our existing solution onto Office 365, I'm not really in any hurry to go through that experience again! Is there a quick, automated way to move all of our Exchange information from our existing plan to a new plan?
A1: For user info, you can refer Add multiple users with bulk import to bulk import users by creating a .csv file. For the items in Exchange Online, you can use Outlook desktop client to export data from old subscription, then import them to the new one.
Q2: Is there a way to do this without running the two plans side-by-side for a month? I don't want to pay for a month of P1 license when we're just moving information on one day.
A2: It’s not recommended to do that, since if there is anything wrong with the new subscription, there will be no backup plan for your organization if you canceled the old one already.
Q3: What is provided? Do you just receive a license key for each user?
A3: You’ll need to assign an Office Professional Plus license to the user first in MOP(Microsoft Online Portal) before the user can use it.
Q4: How is this provided? Does it show in a user control panel somewhere? Is it emailed through?
A4: You can install Office desktop client via “Downloads” link in MOP.
Q5: How do you deploy the software? Is there a network installer? Is there a cd image download?
A5: You can let all of your users download from MOP one by one, or you can also download the source file from there once and then create a deploy file yourself for your organization. For more details, please refer to: Install Office Professional Plus with Office 365.
Q6: If license keys are provided, are they revoked when you delete a user? Or do they go back into a "pool"?
A6: If an employee leaves your company, a global administrator or user management administrator can delete that employee’s account. When you do this, any licenses that had been assigned to that person become available and can be assigned to other users. For more details, please refer to: Manage licenses.
For more info, please refer to the links below:
Connect Office 365 to your Outlook desktop application
Outlook Data Files (.pst and .ost)
Export Outlook items to an Outlook Data File (.pst)
Import Outlook items from an Outlook Data File (.pst)
Please feel free to post your updates here at your convenient time.