I also received Diners Millennia LTF to DCP last month and still the offer is available but I am not going to upgrade it as DCP is scrap now.
I mailed to hdfc to upgrade it to Regalia Gold or DCB but they replied it is not possible right now. My Millennia CC have 2.2 L limit.
It is good to convert Diner Millennia LTF to Millennia VISA variant to get upgrade offer for Regalia Gold to Infinia ?
I also have Preferred family banking with 3 accounts with maintaining around 8 lac average balance in all accounts since Jan 2023.