The hubs are interchangable, but not the same. The early bikes have 8mm bolts (12 or 13mm heads), whereas the 125C has studs (can't recall if they're 8 or 10mm, but I think 10mm). Nothing that a few minutes with a drill can't fix... (or the correct rear sprocket if you're going the other way).
Rim offsets are the same, I think. I know that the 125D+ wheels and the monoshock DT wheels have a different offset, but I'm pretty sure that the earlier ones are fine.
Backing plate is definitely interchangable. Theres a ton of different part numbers, but all of the twin shock 125/175s and YZ125C/X brake plates are interchangable.
Axles are all the same diameter, so no drama there.