Another update: AI changed the flight to 1420 from 1320, and now with a 788 instead of 321.
Any attempt to view the booking now has:
At least one of your flights has a schedule change.
and cancelling it online is not possible any more:
Changes not permitted online
Cancel & refund not permitted online


. On the positive side, the S2 flight arrives in CCU at 1120 so now I have plenty of time to cancel the AI ticket before the T-1h deadline in CCU.
If I manage to catch the S2 flight, else its a 6+h wait in DEL with two tired kids. The Mrs won't be amused much either.