Workhorse Allison Jumped from 5 to 6 Speed (on its own)

I have a 2010 Bounder 33u on a Workhorse Chas is with Vortec 8.1 and Allison 5-speed (approx 26,000 miles). I've read that the 5-speed can be changed to a 6-speed but this is pretty strange. We've be...