That was the original plan, to replace the 180 units first. The introduction date was going to be 10/07/17. Then it was double units on the HST diagrams that would have ended as requiring diesels with the 9 car electrics coming in last on the HST South Wales and Bristol only diagrams. May 2018 when Swansea was first due to be switched on was crucial in the electric IEP introduction.
The NR infrastructure issues saw that plan ripped up late last year. The 10 cars will now start first, on the Bristol and South Wales HST diagrams, first day in traffic was 24/07/17. The 180 sets would go with the December 2017 timetable (replaced by 5 car 800 sets - 4 diagrams) and the 9 cars, now bi-mode, would go on the HST diagrams that work the North and South Cots. 10 car working on those lines is out, till at least Dec 2018.
At every timetable change the HST diagrams are altered to suit the incoming 800 units, which have a completely different maintenance regime to comply with. That means it isn't one in, one out as you need more long 800 units diagrammed than equivalent HST sets to comply with the TARA maintenance regime which requires longer downtimes than the HST sets. As you have to operate long trains in the pre Dec 2018 timetables (apart from the existing 180 workings) you can't cheat on this by going load 5 vice an HST.
The 24/07/17 date is now pants thanks to the GW test section being delayed. Agility has told the DfT that the units (both GW and EC) will now be delivered at least 3 months late because of this (apparently they can do this in their contract) and this is causing everyone grief as the HST cascade to Scotland means these units have to enter service on GW during 2017. There are some high level discussions going on at the moment to try and sort this king size mess out.
Incidentally, going "full chat" on the IEP engines may sort out some of the potential timing issues on the diesel sections but the fuel consumption goes through the roof as a result. That affects the ability to complete a days work without having to go to depot sometime during the day, depending on the diagram. That is another issue that is being currently looked into as well as the effect of having to slightly decelerate some existing HST services in advance of introducing 800 units on them.
The only current good news is that any potential delay to the Agility units doesn't affect the 802 deliveries - that is a different contract, with Hitachi only and FG are in full control of that one.