Good question, and one to which I can’t find the answer. For EMD, the issue would have first arisen with the DDM45 D-D locomotives around 45 years ago.
My best guess is that the traction motor groupings are 2S4P and 8P, with field shunting in 8P, perhaps also in 2S4P. Also possible is fixed 8P with field shunting. For example, EMD used fixed 6P with field shunting on some narrow-gauge 6-motor locomotives, even in the pre-alternator days.
On that basis the 8-motor locomotives at interest would not have more transition steps than “normal”. And as you say, that is because of their use of narrow-gauge motors. The current draw of 8P narrow-gauge motors in any situation is probably similar to that of 6P standard-gauge motors.
But the foregoing is speculation, so we need some “hard” information.