I emailed Pandora customer service and this was the response that I got:
Thanks for writing. Sorry for the confusion. There was a bug previously which had allowed listeners who had Pandora on earlier Sprint phones to get Pandora One for free. Because Pandora is free on all supported Sprint phones, this shouldn't have been the case.
We've now fixed this bug and unfortunately you won't have access to Pandora One at this point (without upgrading for $36/yr). Please indicate that you want to use Pandora on the web, if you plan to continue listening for free.
Note that you can also listen (using the same account) for free on any supported mobile device - listed here:
www.pandora.com/on-the-goHope this helps. Thanks so much for listening!
So basically our access before was due to a bug and eventually all PandoraONE access will be revoked. Hopefully we should still be allowed to listen ad-free but I don't know about that yet.