Thanks again, John. After reading all of the responses, I'd been kinda bemoaning the fact that I hadn't gotten out and shot anything with the modified numbering. I did some more digging through the negatives from that period, and...
I did! I still haven't gotten through everything, but I, at least, have shots of "UP 3718", back in Pierce again on its way back to Cheyenne. I feel a bit like a kid at Christmas, as I didn't realize I had any of this. In fact, I sorta wonder if I'd even noticed the number change at the time. It seems like the sort of thing I would've remembered.
This isn't the best shot of the series, but it's the clearest showing the numbering at this size.
Thanks again all!
Scott