Previously if you were an Office 365 (E SKU) customer or partner with an Enterprise Agreement (EA) you could ask MS to upgrade your Yammer network to the Enterprise edition for no additional charge.
Starting today MS have added the ability to do this via the Office 365 admin portal directly.
This is a rolling release, and your tenancy might not have got the changes just yet. So hold tight.
Customers with Office 365 E plans, SharePoint Online + Yammer, or existing Enterprise Agreements and qualifying license purchases are eligible to activate Yammer Enterprise via their Office 365 Administrators portal.
You can do this in the Admin portal as shown below:
This should make the process a whole lot smoother and is another (small) step in the right direction for the integration of Yammer and Office 365.