TfW have told me that the 150s are staying put on Rhymney - Barry - Bridgend services until the 756s are in service on Rhymney services, which is dependent on the 398s entering full passenger service on the TAM lines.
So the 150s have at least 18 months left on Rhymney - Bridgend. And today almost every Caerphilly to Barry/Bridgend service is a 150.
To quote a "normal" who I spoke to last week "Why do I see all the new trains going to Barry Island, but whenever I get on a train to Llantwit it's an old diesel train?"