It is unfortunately not possible to transfer the Premium plan to other accounts.
Then via the Google Play/App Store, please log in to the app you purchased the subscription with while using the account you want to switch to and then proceed with restoring your purchased items.
You can restore purchased items from home screen of MediBang Paint app.
Look for “Restore purchased items” or “Verify Current Plan” on the sidebar, tap it, and then select “Verify your MediBang Premium Status.”