Some portions of the interior were altered in the sense that they were removed or rearranged, but the items would stay the same. For example, the entire radio operator/gunner position was deleted along with the front hull plate in place of the angled glacis and armored hood...but the location and size of the driver's position remained unchanged (driver's position always on the right side in the 38t vehicles except the Hetzer). The engine compartment was shifted to the front but the engine and transmission elements and design remained the same. See diagrams below from the MBI Publications "Marder III and Grille" by Vladimir Francev and Charles K. Kliment (excellent resource btw on the H and M variants). This means that the parts from the earlier Pz 38(t) kits are just fine for what would be retained in the Marder III M chassis.
And, just for the curious, an interior shot looking out of the driver's hood from the old AFV-Interiors site that went away some time ago but whose text that accompanied it stated:
"I have been intrigued by this interior photo of a 38(t) variant for many years. It comes from one of the large gunned versions of the 38(t) but provides us with some useful information about the driver's area just the same. From approximately the visor area down, the controls are the same as all 38(t) variants. The transmission is to the left and you can see the unique levered steering tiller linkages crossing to a handle to the left of the driver's legs and a second handle just to the right (here caught in the shadows and somewhat difficult to see). Down below are the brake, clutch, and accelerator pedals. The gear shift lever is to the left, in front of the steering lever pivot, directly on top of the transmission. Also in view, to help identify the vehicle as a 38(t) type, is the communication box device at the front right corner of the hull. The box has a series of colored lights that were wired to a similar control unit at the commander's station and allowed a simple visual communication between the two. The parts of the photograph that intrigue me are the apparent open over-head hatch and the holed support structures to the left. My best guess is that this photo is taken in one of the Panzerjager 38(t) variants, perhaps the Marder III, Ausf. M."