Originally Posted by ExpatSomchai

First question - Will you have checked bags? If not you may be able to get away with it without any problems.

Second question - Are you willing to lose all the Avios for the flights taken? BA have stated that they are clamping down on this.

My suggestion would be to go to check in for the return leg. Explain that instead of going to X you need to go to Y, produce your new ticket and see if they will accept.