Should you replace the transmission fluid on an Allison transmission?

Depends on what type of fluid you have in your transmission. If it has Allison's transynd in it, I would typically change the spin on filter once a year, then send an oil sample in to Allison through an authorized Allison transmission dealer. The results of the oil analysis will tell you if the oil needs to be changed, as well as if you have any kind of wear materials from any kind of problems developing inside your transmission.