At the time the "same" engines were being taken off the 400's and put on the 767's as the 400 engines were of a later mod state. All to keep the figures looking good.
Factually incorrect.
767 engines were being exchanged for engines on the 747 for sound engineering reasons. Nothing to do with the 'mod. state'
The explanation is long and irrelevant to this thread or in fact to anybody outside the company.