As a quick update - I called the Premier line to check on whether it is possible to make this change and was told it would be possible provided there was P class available.
I don't know if perhaps the rep was misinformed, I did ask them to confirm with someone else and the same answer came back - yes.
My itinerary is EWR-IST, how I get there can be SDC'd as long as it's within the SDC rules and the feedback I got was that changing from EWR-XXX-IST to EWR-IST was just fine.
We'll have to see whether it pans out that way tomorrow.
As an aside - star-world, I meant to harm. Perhaps you are right and I shouldn't bash UA. Sometimes they do a good job of provoking me though and venting about something unrelated - eg. SDC - is the outcome.