Hi Roadrunner,
This depends on the cabin of travel, and the country of origin.
The best thing would be to ring the contact centre and ask.
If you purchase 'likely' dates of travel on a lower, restricted fare, then, provided you travel OUTBOUND on the date you booked, in MOST cases, your return, if you change it, will just be for the required change fee, subject to availability of the booking class your ticket was booked into (ie: Q, B, L, M, S etc)
If your ticket were a UK originating Upper Class fare, then the only option you have for flexibilty is the full fare 'J' ticket, so it could be booked as OPEN or as an actual date.
Bear in mind that OPEN does not confirm a date, and is still subject to availability. Sometimes if you want to use it on a busy flight you cannot, unless you travel standby. (that is only if that flight is already sold out)
Hope that Helps,
Juliet