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.