My company is a cloud champions partner and we sell tons of O365 seats, it's been a frequent complaint from people that get the small business SKUs and then want to add on Enterprise SKUs such as exchange, lync, kiosk, CRM, etc and they can't. It's also very painful to migrate from a small business or mid size sku to an enterprise one, it actually requires a full migration, not just a license change.
We've had this conversation with out PAM a few times, and we're told that the O365 team is looking at making it easier to upgrade/downgrade licenses. However, we have heard nothing about a change to allow mixing of the various SKUs. I kind of doubt that will ever happen, mainly because the small business SKUs are heavily discounted.