When I purchased my ticket I was told I could change my return date for no charge so long as the flight had an open seat but that the ticket needed to be used in three months. I am considering extending my stay in SE Asia for a couple of months past my current return date, but if I do that it will put me over the three month mark. Is it usually posible to do this after three months for a fee or would my return ticket expire and become useless, so that I would need to purhase another (one way) ticket when I wanted to return home?

Thanks to anyone who can shed some light on the subject!