I hate to say it, but I think you'll need to go back to the orginal TA.

I'm confused. We've changed agents in the past, but even so, the original booking agent is responsible for your booking as you are part way through a trip. Or have they made some provision to transfer control of your booking to a new agent.

I can see BA's reluctance here. Ever since they slashed commisions, BA and TA's do not get along. Call BA: You need to speak to your agent. Call TA: You need to call BA.

Other question: Since it is such a discounted ticket, does it even allow changes?