An update on Yammer will link the network name (company network) as well as the default domain with Office 365.

This means if the Office 365 tenant name (not the URL xxx.onmicrosoft.com) will be updated, it will be reflected automatically to Yammer.

image on Yammer will become image from Office 365

Also, when you change the default Office 365 domain to a verified domain (which should be already the case for most of you), this domain will automatically becomes the primary domain for Yammer

The update is scheduled to be effective on November 14.