I am slated to fly on UNited from CCU-DXB-EWR-ORD-DSM. The CCU-DXB is on Emirates (EK). The connection at EWR is too tight for my taste (1:23, I am surprised UA gave me a ticket on this connection). So, I would like to consider CCU-DEL-EWR-ORD-DSM. The CCU-DEL leg would be on Air India (AI) or Vistara. All other flights are on United metal. Would a SDC (same day change) be possible here? Thanks!
No. Same-day change is only available on United-Metal flights.
Once you are in the air on the first leg you might be able to use Same-day change change the later flights (using the United App), although given your itinerary it's unlikely that any alternatives will be available.
1 hour 23 minutes is a tight connection, but it's also very doable depending on the queues at immigration - especially if you are a US citizen, and even more so if you have no check baggage. If you do miss-connect in Newark, United will move you to a later flight to get you to your final destination.
In your shoes, unless you really have to get to Des Moines on that specific day, I would do nothing and try my luck with the connection they've given you.
If you're lucky, you'll make the connection, although I agree 1:23 is awfully tight for an international to domestic transfer anywhere in the US. If you're not, United is responsible for getting you to DSM by some other route, and there should be plenty of possible connections from a major airport like EWR — although depending on time of arrival etc etc you might get delayed by up to a day.
Also bear in mind that regardless of what flights you originally book, you do have four (!) connections you need to make along the way, so it would be wise to allow some buffer in your plans in case something does go wrong along the way.