I rebuilt an N-scale 2x8 layout twice. The original was a very basic toy train double track loop that followed the edges of the layout (started Thanksgiving day 1974). Summer of 1976 I redid that into one that was a little less boring by changing it to a single track mainline with passing sidings. What had been the interloop was squeezed into more of a water wings type configuration. With a yard in one of the loops (once again another toy train type thing), and a reversing cut off at the tight point. Then in 1981? I had the inspiration and went with elevations. The ridiculous yard went away. The inside loop was reworked to be an uphill grade dead ending at a large mine. The passing siding became a junction instead. The front of the outside loop was curved to the rear (getting rid of the tracks following the edge of the benchwork) making it a water wings sort of arrangement. Industries then populated the space created there. The rear outside loop remained where it was, but I added an additional siding (3 up), put up a barrier so it became hidden holding tracks. At the time I thought I was very clever, but later found out the concept of hidden storage tracks and the layout as a stage or scene had already been published. sigh - seems I am always one step behind the cutting edge.
Other than that I cannot think of any time I redid a layout. The other eleven or so (not counting modular units) were either sold, razed, or I still have them.