One most important point bro, HDFC issue only one Core Card like your Moneyback or Millennia and utmost two Cobranded Floater Cards like Swiggy and Tata Neu. Now coming to your query, if you work in IT giants or similar MNCs, then Millennia should be LTF through out the year, you can Close and Reapply after 3 months. If not, then use Moneyback for sometime, increase limit atleast between 50-80K , keep an eye for upcoming LTF Millennia offers. Then close and apply through agent. Also, don't take cobranded floater until and unless you have core. Take core first.
Like, if you are planning to close Moneyback, then take floater after getting Millennia or Diner's Privilege. Why I'm saying this? Bcoz, say your are taking Swiggy now as you have core. But when you will close Moneyback and in between cooling period, you will be only holding floater cobranded and you will face issue in getting core like Millennia. This rule doesn't apply in case of upgrade.
Hope this helps, best of luck.