Rebooking outside the 3 month validity period of ticket

I have a round way ticket which states that I can stay for maximum of 3 months (which is also the ticket's validity period). I wanted to stay for four months and rebook my return ticket. It is stated that if I change flight within the 3 months validity period, they charge USD 81. How about if I rebook it outside the 3 mos validity period? How much do I have to pay?