Office 365 online help - premium subscription how add user with just email?

Current State

I have a friend with a small business that's just changed the business name. They use Office365. They have the email address manager@example.com and the subscription gives them Word / Excel / Outlook.

Target State

They would like their new domain example2.com to be the main domain. They want manager@example2.com to be their main email address, with all email to manager@example.com going there. All documents from the old manager@example.com should be available. No email in the current account can be lost.

What I've Done

I've set up a website on SquareSpace, and email is on 1st domains for now. I've added the new domain example2.com to Office 365 and validated it. I've added a new user, but left them without a license because it's $20 a month. They don't mind paying that for a few months if it takes a while to set up.

I've used Google for Business plenty and understand it fine, but know nothing about O365. I've done plenty of email migrations to Google Business email.

Question

How do I get the new email address manager@example2.com working with a subscription for email / Word / Excel / Outlook, with all the email and any documents and such from the old manager@example.com still available, without paying for two accounts for an extended period?

I know I'll have to set up email forwarders / aliases, that should be easy. The bit I haven't worked out is the document migration, or perhaps how to rename the current account.