O.k., I've found a reference book: "How Diesel-Electric Locomotives Operate: the last 25 years including ACs," by Dr.W.J. White, D.B.A.(*)
According to it(**), all 40 series (both plain and Dash-2) FOUR-AXLE locomotives were set up for permanent parallel operation: no transition. Six-axle 40 series units DO make transition, at the motor level: SD-40 and SD-45 at low speeds have their motors wired into three parallel groups of two motors each, and at higher speeds make transition so that all motors are in parallel. SD-39 and SD-38-2 (the book makes no mention of plain SD-38, and seems to apply only to SD-39 with AC/DC transmission), units intended for low-speed lugging, start out with motors wired into two parallel groups of three motors each, and when they speed up to not-quite-so-slow make transition to the configuration SD-40 and SD-45 use at low speeds.
(Dutch, this is why, I assume, the others asked about your qualifications: in passenger work, and maybe also in your earlier work in a dieselshop if it was in ex-NYC territory, you would have had experience primarily with four-axle units. As long-time readers of Railroad-net forums know, you are generally knowledgeable, but you seemed to be denying their experience of transition on SD40-2, so....)
Apparently the change with Dash-2 was that plain 40 series, before making transition, went through several stages of "shunting" (field weakening), and this was eliminated (except on the SD38-2) in the Dash-2 models. (Hazen comment: even if this is true in general, I suppose there might be exceptions: units built for customers who thought they had special needs.) And apparently transition "at the alternator level" came in with the 50-series. (40-series-- both plain and Dash-2-- used the AR-10 alternator. 50-series and later (and also the small number of SD40-2SS built for BN and UP) used newer alterrnator models-- my guess is that alternator-level transition was only possible with the newer stuff.)
(*) Copyright 1996,1997. Publisher identified only as "Peat". No ISBN that I can find. Address for enquiries: PEAT, Attn J.D., 1001 Pearce Dr., Mansfield, Ohio 44906.
(**) pages numbered separately in each chapter: the information cited is from pp. 2-6 to 2-11.