Currently, I'm holding return e-tickets from my home (A) to a holiday destination (C) via a former home town (B) for Christmas/Scottish New Year. This is in Europe (A) and Asia (B & C.)

So My trip currently booked by two separate transactions on an on-line agent is

A to B to C to B to A.

My employers have recently changed their holiday conditions and my outward date of travel now breaks my employment code.

I would like to buy a an outward single A to C for a few days later discarding the current outward A to B and B to C. There seems to be nothing displayed by the agent's web page conditions disallowing this but the reservations clerk on the carrier for A to B and B to A, says that B to A will be cancelled if I don't turn up for A to B.

Is that correct? Could I sue the carrier if they refuse to honour their e-ticket. Have I undertaken to attend the outward flight?