Will fenders interchange between 1972 and up Dusters? If not, what are the differences. The style lines look the same to me.

I'm fairly certain the generations that interchange (physically) are 70-72 and then 73-76.

'73 thru '76 I can tell you with absolute certainty interchange without problems.

'70 and '71 have the square, flush marker light while '72 only has the "corporate" plastic marker.

Dart & Dart Sport or Demon fenders do not work. Plymouth with Plymouth, Dodge with Dodge. Unless you want to make a Plodge or a Dartmouth?

Ramblerman is on the money here.
The 72 Duster/Valiant fenders are a one year deal. The last to utilize the steel panel that supports the grille, and the first to use the corporate side marker lens.
In late 75 or early 76, the RH fender was slightly different around the antenna hole.