I have this exact same problem. I pay for Pandora Plus through Google Play. Pandora sends me to Google Play to upgrade, and Google Play sends me back to Pandora. Neither offers a working upgrade option.
I do not want to cancel my existing subscription or lose the money I've already paid.
Please open a support ticket for my account and help me upgrade to Premium with credit for my unused Plus subscription.
@mango6965 Sorry for the trouble you are having upgrading.
I have created a ticket with our support team for you.
They will be contacting you privately via email to help look into this further, so please keep an eye out for their message.
Thanks for your patience in the meantime!