Changed Office365 password, cannot change in mail.app: account already exists
I had the same thing happen to me this evening after updating my Office365 password. 

What worked for me was to open my desktop Outlook App and send myself a test email. Outlook was then prompted to ask me to put in the password. After adding the password to Outlook and sending the test email, I then went back to my Mac Mail and received my test email there. I also checked the Outbox in Mac Mail, which had three emails waiting to send. I double-clicked on the first one and then all three emails sent.