Originally Posted by
G-BOAA
Changing the routing removes the choice to be able to make that stopover in SIN.
No it doesn't. You're probably on a fare than offers the option of a stop in SIN or BKK, or you can add a stop by upgrading to such a fare.
BA putting you on a via-BKK flight doesn't stop you changing to SYD-SIN(stop)-LON, providing this is available. But if for example BA cancelled all via-SINs, or they sell out, BA doesn't have an obligation to keep the SIN stopover option available to you while you're booked direct.