If you are paying for a subscription through Google Play, you should be able to use the following steps to get your subscription activated and returned to your new account:
Open the Pandora App on the Android device you used to upgrade, and log into the desired Pandora account.
FromMyCollection, go to theProfiletab in the bottom navigation bar.
Select Upgradein the top left corner.
Finally, select Retrieve your subscription.
This should reapply your subscription to your account for you.
This all being said, if someone else still has access to your old account, it may be worth canceling your subscription through Google Play entirely, and then resubscribing directly through the Pandora website instead to avoid any future issues with that.
Let me know if you'd prefer to go this route, and I can get you in touch with someone from our support to assist.