IIRC, it needs at the very least a tear down, inspection, and recertification of the boiler. That was one of the major reasons it was pulled from service in the Heritage Fleet. Once the Big Boy was up an running, they had a "super locomotive" and mothballed the Challenger.
I got to see the Challenger both up-close and under steam back in the early 1990s when I lived out west. It stopped overnight in SLC on an excursion run between Omaha and SF. I went down to the UP station in the afternoon for the tourist time and then parked out along the tracks outside of town to see it depart. Don't know where my photos got to.