Reportedly in February 2016, Microsoft began to automatically upgrade all Office 365 customers to version 2016. One has to search for the words “February” and “automatic” in the page below to see it:
https://support.microsoft.com/en-us/kb/3097292
Among many others, this can cause major problems for users of QuickBooks of all versions before 2016; all integration with Excel fails, and we can expect other problems (e.g., Outlook integration) as well. There is no easy revert procedure, we just have this:
https://community.office365.com/en-us/f/172/t/409681
There is a method to prevent the upgrade via Group Policy: