Google doesn't currently provide a way to transfer paid apps from one account to the another.
Still, you can add your "old" account to Android Settings => Accounts on your device and select this account in Play store app (side menu => right top arrow next to Google account), then restart Poweramp.
If you purchased on Poweramp website:
Please just send us your the new desired Google or Email account. Please send it from your previous Google/Email account or email used for the purchase so we can know the request actually comes from you.