Again I’m not formally trained on IET traction; my professional interest is only in 16x and 387s - but I always got the impression that with the planned fully-automated changeovers what you suggest was at least in the specification - the train would “know” it was approaching a changeover and fire up the GUs a few miles beforehand to enable a seamless transition from AC to Diesel operation.
My last (and so far only) IET cab ride was when they were very new to us; and the power changeover (at Maidenhead) on that occasion was completely manual - the GUs didn’t fire up until commanded to by the driver. I didn’t ask at the time if that was the normal way of doing things because frankly everything was quite new, and I didn’t want to sound like I was questioning what the driver was doing!
@Mintona is a lot more conversant in the ways of the IET than I am - perhaps they can lend some insight as to whether it’s all automated yet; and if the GUs fire up automatically beforehand...