We have legacy users that need to be able to use the same template built with building blocks. Some of these were built with 2010 and need to be updated by adding building blocks that got deleted. If the template is opened and updated using 2016 what will happen if saved in 2016 and then downloaded from the SP website to a system that has 2010 on it?

AFAIK, there is no difference in how Building Blocks work in Word in any of the Ribbon versions. That is to say that the Building Blocks saved in a template in any of these versions will function in any of the other Ribbon versions.

There are differences in the Cover Page built-in building blocks. There may be other built-in building blocks differences that I haven't noticed. These should have no effect on building blocks stored in your custom templates.

Automated Boilerplate Using Microsoft Word
Where can Building Blocks be stored?

Do you mean that all of the Cover Page BB's are affected? What about custom built ones? What type of differences? How can you tell there are differences?

I mean that Word 2010 and 2016 have different built-in cover page building blocks. Custom ones are not built-in.

OK, thanks.