Called AC asking about the change, and as expected, agent simply told me "no problem, total cost around 4000" even though there is no actual fare difference in the return leg I'm trying to delay by one day. I didn't proceed further, just said thanks and I would call back later.
This day extension is not mandatory and I will not pay such an amount for it.
Lesson was simply to not book the refundable fare on one leg, money lost.
[MENTION=791802]Bartolo[/MENTION], hope all works out in your situation. warm regards.