I'm not so sure they will all have gone by 1st Jan 2020.
First of all, number of units is important to consider alongside number of vehicles although without knowing how many doubled-up diagrams there are it is probably going to be hard to come to any really useful conclusion. You say there are 290 vehicles, Wikipedia says 10 are 3-car units, which leaves 260 vehicles or 130 units. So, without that info on multiple working we have to assume we would need to find 130x 2-car units and 10x 3-car ones to be able to withdraw all the Pacers. I think Pacer vehicles are the shortest on the network, so any replacement unit will have a little more capacity, but other routes need units for extra capacity also.
Then there's also Transpennine and Valleys wiring, although again both these will be tight for 2019 given the delays we are currently seeing on electrification projects, and given the current uncertainty over the latter.
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I don't believe any of the owners will be willing to invest in the work required to prolong the life of the Pacers. So the market will decide. It would also take a very brave government to risk the wrath of the northern population if its clear that adequate volumes of rolling stock is available elsewhere.
As far as I'm aware from reading various sources, Angel expects to scrap their class 142s (that at least comes from Modern Railways) but one of the other
ROSCOs» (Porterbrook I think) has developed proposals for TSI-
PRM▸ compliant units (143s and 144s I think). The ValleyLines project has always been unclear, with some sources showing it to be done in the control period leading up to 2019, but others saying it is to start in 2019/20 for completion in 2024. And the government can always protect itself from the wrath of the north by saying something along the lines of "we are having to make spending cuts and it was not considered affordable to find replacment trains for all the Pacer fleet so we had to retain them or we would have been forced to close lines".
Thus, my guess at the moment is that we will see most of the 142s withdrawn by 2020, except perhaps the Valleys units which might be given a derrogation for a year or two to allow enough of the valleys to be wired to get rid of them. 143s and 144s get TSI-PRM compliant upgrades and run for maybe another 10 years (depending on how long is required to justify the ROSCO's investment). Unless this ex-tube-stock
DMU▸ proposal comes to fruition, in which case maybe the Pacers will all disappear.