Bill,
I imagine you are probably correct that the three engines spared were the ones with the most time left on them.
At least two of the K-36's (one was 484) were fitted with steam lines for use on the San Juan. The K-28's were not required to handle the train by themselves.
The San Juan was discontinued in January, 1951, by the way. Before that time, engine #478 was already fitted with a power reverse, and like #473, was destined for use as designated switching engines in Alamosa. Following their time on the San Juan, 476 was used in freight and branchline service. 473 and 478 were primarily in switching service, with occasional forays out on the line as needed. But the "official" designation of 473 and 478 was as switch engines. They were originally intended for such service at Alamosa, after the retirement and sale of 463 in '55, one of them or the other probably spent more time at Durango.
D&RGW management was able to foresee that 10 K-28's were not needed, and most were expendable.