I have drawn up a rough sketch in Paint to depict what I think would be the outcome -
The Shtil arm-launchers make use of a good deal of below-deck space to store their ready-to-launch rounds of the 9M317 missile (only 1 round can be carried on the launcher at a time). The removal of the Shtil system will free up all that space. In addition, structural modification to a certain extent is again possible and cannot be ruled out (especially given the fact P-15 could be modified to accommodate the the twin Barak-I VLS units (storing 32 missiles total) in the middle of the ship.
The area around the arm-launcher in the rear of P-15 has quite a large space there.
However, there are still some unanswered questions regarding the upgrade -
1) What will happen to the 32 Barak-1 missiles? Will they be retained on the lines of SM-3 and ESSM operating side by side? Or will they be thrown away because the new P-15A design dictates that a ship with Barak-II no longer needs Barak-I (the Barak-1s were added because IN thought 9M317 wasn't so good at intercepting sea-skimmers, which are only detected at close range. Barak-II is designed to have superior performance against both high-flyers and sea-skimmers)?
2) Any RCS-reduction measures in the offing? I don't think it would be a very feasible upgrade WRT to RCS because P-15 was never designed to be low-observable from the outset, but it does not hurt to know IF there will be any form of methods employed to reduce radar, infra-red & acoustic signature of the ship?
I have some more questions that are still waiting for answers in my mind, I will ask the said analyst about the same soon...and let PDF members know the same.
Also, I request TTs like you and others that are in touch with IN to do their research from their side as well and see if they can dig up something about this.