They swapped the whistles the Sunday before the Fall Spectacular, and replaced them afterward. Just a one-time thing. With all the changes happening around the EBT recently, it would be cool if whistle swapping became a new tradition, so we could hear some of the whistles that haven't sounded in some time. Btw, there was also a noontime whistle salute consisting of 15(17), M-7, and M-1 after Mr. Kovalchick and Mr. Salone made their speech on Saturday. I don't think there's been one of those since 2001 or '02.
Here's a video the EBT's hostler took on the day they changed whistles. 15 is mostly painted black, except for the tender wheels and hiding the boiler bands:
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