Tickets are valid for one year.
If you purchase a ticket on the 27 April 2015 then the latest the outbound date could be switched to travel on would be 26 April 2016.
*******Note 27 April 2015 in this example is the purchased date not the date you book for travel*********
This cant be extended.
The inbound can go further ahead but needs to be within one year of the outbound.
****** all opinions are my own and do not represent ba or iag*****