Without any actual experience of Air India, but your e-ticket is an important document for your contract if carriage, and your luggage allowance should be shown on there based upon the fare you paid. So unless your ticket has been reissued and modified the luggage allowance should still be as booked, even if you already know there's a risk your aircraft has been swapped to all Y.

If you get a luggage query at check in just gently refer to your e-ticket, and if the agent still says no, ask for their supervisor for a second opinion. At worst case pay the excess, get a receipt, and send it in to customer disservice for a refund after you fly. You paid for the luggage allowance so assuming no reissue or refund then you should get that allowance.