H1B visa appointment with existing petition when new petition is pending/approved

We are planning to go to India in April. We currently have our petition approved till August 10th.Our extension may be filed in March(planning to pay for premium processing). If we fill ds-160 using existing petition, but our new petition gets approved before our visa appointment, do we need to fill new ds-160 and pay again? Or can we use existing appointment and take new approved petition for the interview?

You just fill and submit new DS-160 referecing the new I-797 and update your appointment with new DS-160 or just call VFS and they xan do it for you.
You dont need to cancel your appointment or pay the fee again.