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.
on Yammer will become
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.