Stan Brown wrote:

Windows 10 gets all the publicity, but there's another potentially
unwanted "upgrade". Many of my customers have reported that when they
run a repair of Office 2013, it changes to Office 2016. In theory
that's a good thing, as Office 2016 is slightly less annoying than
2013. But older add-ins that worked on Office 2013 may not work on
Office 2016, and then the customer is forced to do without them or
pay for upgrades of those add-ins.

Since I have never heard of a free upgrade of Office anything, you sure
these customers have a paid version of Office 2016 (perhaps due to
Office 365) and not a trial? Could've been some bundleware they
installed with something else they intended to install. Did they
actually have Office 365 and got the Office 2013 local apps that were
available back then and now they let Office 365 upgrade to the newest
versions (2016) of the local Office apps? The point of subscribing to
Office 365 is that it will include the latest Office apps.