May take this with a pinch of salt, but I've heard this from several reliable sources, and now seen this tweet attached saying tha possibly different traction to the 196 will be used on the EW line.
I've received a DM from a senior union source, saying that the union has heard rumours that a different type of traction may be used on the East West route, as opposed to the 196s. It's expected to be discussed at next week's meeting with Chiltern.