Call hdfc customer care.
But DCB is a paid card with 10000+gst annual fee.
I also have a regalia card with greater than 10L limit since the past 7-8 years.
I also tried to upgrade to DCB last month. They said it was possible but I will have to pay the annual fees (even though I was a long standing member with a very high credit limit) and they will not give LTF.
Just try your luck. Maybe you will get lucky.