The FAQ says:
As a SERO/Everything Plus customer, are there restrictions on where I can upgrade my device?
Yes. You can upgrade through any of the following channels: any Sprint-owned Retail Store, Telesales 888-882-4030, or
Sprint.com. You will not, however, be able to upgrade your device through a 3rd party dealer or national retail partner and you will not be required to provide a referring employee's email address and CID in order to upgrade your device.
I would think this would apply to upgrading to premium as well, but who knows...