Unless there are strict commitments to get rid of the existing stock by a certain date (seems unlikely at present given how much of a sudden surplus there will be, what with almost all of it being AC only) I don't think there will be any major catastrophe with the existing stock - but I do think it's almost certain it won't all be introduced as quickly as the DfT has said - as I say, I would guess at it being 12-18 months later than that before there are no scheduled operations for any of the legacy stock - given the higher desirability of the more modern units, unless there is a deal to lease the old stock for pittance (I doubt that based on what's happened in the past), then I think there'll still be some tatty 321s and 317/6s running around in 2021, possibly 2022, long after the 360s and 379s are gone. The current Thameslink cascade is a prime example of this, though I realise there are other factors at play - Electrostar numbers dwindling with 700 arrivals, yet plenty of 319s still sticking around to the bitter end.
Where I do think things could go the most wrong is the rural route - the DMUs are desperately needed elsewhere and will be snapped up quickly. If the Stadler Bimodes, arguably the most unusual mainline rolling stock to have been introduced in the UK for a very long time, do not perform well out of the box, I can see a period where the old DMUs are with their new owners and we have periods of replacement buses due to insufficient fleet coverage until they get them bedded in. Hopefully I'll be wrong about that, of course.
Wivenswold said:
Ah, yes, you're right. That is a serious concern. Perhaps it's time for me to get some info together and press DfT and AGA for details of changes in numbers of peak-time seats from each station. My last response from AGA nicely side-stepped my request for reassurances that 5 car trains won't replace 8 car ones.
Now why do you think that might be?

Replacing the lesser-used 8-car services with 5-car ones is one of the main reasons to choose units in that formation, providing more optimal capacity where 5 or 6 cars would do, but 4 would not. I think it's pretty likely that the majority of off-peak services currently run with 8 will drop to 5.
Just asking for numbers of seats (which is the most likely request to get an accurate response) doesn't tell all without knowing the configuration of the units (e.g. whether a 2x5x22 is used vs. 1x10x24, and what seating config will they use?)
Honestly I'd rather hope to see 2+2 seating throughout on at least the 5-car units, we can probably rule it out for the 10-cars.
There's also the fun gamble with stock numbers. If common sense was being applied, they'd probably be Class 600 for the rural units, 711/0 and 711/1 or 711 and 712 for the Aventras, and 715/0 and 715/1 or 720 and 721 for the StanEx/Norwich units, unless they qualify for an 8 designation.
In practice? Where's my 100-sided die....