I have experienced 3 major EMU rolling stock transitions on the GN, class 312 to class 317, class 317 to class 365 and class 365 to class 387.
Before any class 379s run in service there will need to be acceptance testing (x miles of fault free running for each unit) and driver training.
The class 365 to class 387 transition is not a good precedent for class 387 to class 379. The class 387s came over from Brighton in dribs and drabs, as they were replaced by class 700s on Thameslink, and they were introduced in dribs and drabs replacing class 317/321, which mostly worked simple peak only diagrams. There was then a "big bang" change from class 365 to class 387 on the Fen line services at the summer 2017 timetable change. This was necessary because of the large amount of splitting and joining then being done at Cambridge. The reduced proportion of peak only working and the reduction in the amount of splitting and joining make this approach unsuitable now.
I expect the class 379 to class 387 transition to be much more like class 312 to class 317 and class 317 to class 365 transitions. In both these cases the unit diagrams were split into 3 roughly equally sized blocks, which, for ease of description, I'll call A, B and C. The transition then happened in three stages.
- when enough new units and drivers are available for a third of the service, block A goes over to the new units
- when enough new units and drivers are available for two thirds of the service, blocks B and C go over to the new units, but block A goes back to the old units
- when enough units are available for the full service, block A goes over to the new units for the second time.
For ease of operation it is better if block A does not include the diagrams requiring splitting and joining.