Since the front face and the step face are parallel you can simply use _MoveFace to shift the step up to the front. If you want a single surface follow up with _MergeAllFaces (which as a bonus will also merge some of the objects other faces) - _MergeAllFaces works on this but, in one of Rhino’s quirks, _MergeFace doesn’t [fyi @pascal]. This is probably the quickest solution for the given model.
So does MergeAllFaces tidy that up - because, while it is there after MoveFace, I can’t find it with ExtractSrf after MergeAllFaces has run?
Although if I use ExtractSrf with Copy=No after MoveFace to select the degenerate, and then delete it, I find that the MergeAllFaces will no longer join the two coplanar surfaces left. So it obviously does do something with it.
Very simple hole diameter changes in polysurfaces. Mitch is right that as soon as there is any complexity you are better off deleting it and making a new one.