You can't simply assume though, that everything will be alright. You.may not be aware that as a 221 driver for VTWC, that I can't drive an XC 221 set. (It has differences such as the missing tilt bars and various alterations). It might only be minor, and under BR, it wouldnt likely even be an issue, but the legal world we are in now, means it's just not an option. So a 769 which will have wildly varying driving techniques, an extra mode of power, very different abilities. It will have to be thoroughly tested for approval by the authorities . Mods are even included in this . Research Lime St .That 142 is far from the only thing to hit the blocks there .
In terms of driver training, of course there will be specific training, maybe as a class 319 conversion.
As far as testing the class 769 in terms of it's suitability to run in service, I would expect a risk assessment to be prepared, identifying and new issues, - either from new equipment/modes or modifications that unintentionally affect the existing hardware. As has been posted above, Porterbrook selected the class 319/4 variant that had modifications to improve its braking capability which through intensive service fully- no
overloaded with passengers, makes its reliability/performance a known quantity. It's strange that the BR MKIII body has long been praised as a sturdy and safe design yet there are prophecies of them folding up like the notorious Pacer event at Lime Street at the first opportunity. As a DEMU, the train will also use it's rheostatic braking for most service braking at speed, so the final/emergency (mechanical) brakes should not suffer from heavy use.
Of course the regular detractors here are looking for future jeopardy to wallow in but I suspect that Brush have broken the back of technical issues, and the project will now proceed, albeit delayed.
As far as the new trains on order go, (331s,195s etc.,) they are as yet unproven in service despite their manufacturers' exhortations, so whilst the 769s will be no racehorses, they should at least provide the required service levels.