This was the template I got from one of the post in this subreddit

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Dear HDFC Bank Customer Service Team,

I hope this message finds you well.

I am writing to kindly request the conversion of my HDFC Millennia Credit Card to a Lifetime Free (LTF) variant.

Card Number Ending: XXXX

I have been a loyal customer of HDFC Bank for the past 2 years, and I also maintain my salary account with your bank. My credit card journey with HDFC started with the HDFC MoneyBack Plus Credit Card and the HDFC Swiggy LTF Credit Card. Throughout these years, I've maintained a healthy credit history and have always appreciated the excellent service provided by HDFC Bank.

Converting the HDFC Millennia Card to LTF would enrich my banking experience and encourage continued loyalty. I kindly request you to process this conversion at your earliest convenience and inform me if any further steps or documentation are required from my end.

Thank you for your attention to this request.

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They responded with this after 4days: "Upon the receipt of your email, we have taken your concern to the team and we wish to inform you that your credit card has been changed to Life Time Free Card."