Originally Posted by
wingzing
I had an ex-IST DONE4 booked through BA. ...
Not super relevant in this case; the rules are a lot more liberal when starting in TC2/TC3 rather than TC1.
The OP started in TC1, IST is in TC2.
Code:
16. VOLUNTARY CHANGES/REROUTING/PENALTIES
(a) Rebooking/Rerouting
(1) Prior to departure
(a) Origin TC1
...
(b) Origin TC2/3
Unlimited changes permitted without charge.
Date/Time/oneworld carrier changes are permitted without
reissue provided Origin/destination/connecting points and
inventory remain the same.
-------------
Local service fees may apply
...