If you do a search with only a one-way search LHR-IND, then that's all that shows up. Your fare class has a round-trip restriction probably, so none of the other fare classes show. Try your desired dates with a return IND-LHR sometime in the future and see if S shows up.

Edit: I just looked - they are there and bookable for 23rd. You'll have to pay a change fee plus any fare differential to switch. I assume your reservation is still active - did you try to change the flight online?