What models and makes go from series to series/parallel to full parallel? Haven't run into any of those yet, and I've had my head in a few cabinets, rewired several, and studied many a wiring diagram.
The Alco and EMD switchers start in series, go to S/P and then S/P reduced field (or field shunt). At least the Alco 539s did; the S/P reduced field was an option on EMD switchers.
Bigger Alcos and EMDS, first gen at least, start in S/P, then S/P reduced field, followed by parallel and then parallel reduced field. (The GP-7 was a bit different; as EMD's first real road switcher, it only had S/P and full P, with no field shunting. The excitation was different as well.) Some of the second gen units (C-424 and GP-30, 35 as examples) had the same transition, S/P to P, but with many stages of field shunting.
I'd hate to even think of all the contactors necessary for S to S/P to full P operation; I don't think anyone did it, at least in production units.