I can think of 1. Nate Meier over Hardy. Who else is there?
There's also a difference between a guy like Meier taking over a starting spot from an older player, and a guy like Wadley or Willies doing so. Meier is a junior, while the other two are freshmen.
I'm looking at the depth chart and these are the guys who unseated an incumbent, either before the year or since.
Tommy Gaul (technically unseated Walsh in the reshuffling of the line) - Sr. unseating a Jr.
Macon Plewa over Adam Cox (injury) - Jr. over a Jr.
Nate Meier over Mike Hardy - Jr. over a Sr.
Josey Jewell and Bo Bower over Travis Perry and Reggie Spearman - Freshmen over a sophomore and junior
Greg Mabin over Sean Draper and Reece Flemming - Sophomore over a sophomore and junior
Dillon Kidd over Connor Kornbrath- Junior over Junior
Matt Vandeberg over KMM - Sophomore over Senior (punt returns)
One thing that stands out here is that the majority of the guys "stealing" a starting spot are juniors or seniors. Of the three positions that have seen an underclassmen emerge, two were considered wide open competitions from the get go, as there was no returning starter. The third doesn't really count, since Vandeberg was put at PR either to help prevent KMM from getting injured on special teams or because he's better at ball security than KMM. He's not a dynamic punt returner, not even more so than KMM.
I could be missing one or two positions, but I think this pretty much covers all of them that have seen challengers take a starting spot.