Either SDC or a regular change would require inventory in your current fare bucket (X/XN for economy, I/IN for business, I'm guessing) to process anyway. When I say you'd have to pay any difference in miles -- UA has been adding between ~2500 and ~4000 miles to prices for close-in bookings. So if inventory opens in your fare bucket, you'll be able to change to it, whether today, the day of travel, or anytime in between -- but you may need to pay this relatively small number of additional miles for the privilege.

If it were an all-UA operated itinerary, SDC would likely be an option, but would still depend upon fare class inventory. The DEL-CCU segment is the problem here.