Adding Echange Online to an existing Domain

Hello all. Our O365 already has a verified Domain. When setting the domain up, we were only using it for Lync. (Online anyway), while keeping email on-premises until later. Well, it is now later and I am working on Migrating email from on-premise Exchange to the cloud. 
Since my domain is already verified on the cloud, can't I simply edit the "domain purpose" to include Exchange online? If so, please explain how. I do not see an option to "edit" the domain.

Thanks for your help in advance