Not familiar in the least with these Fodens, but:
Do these transmissions have a lock-up facility? Many have a system whereby once the drive is smooth the torque converter goes into 'lock-up' i.e mechanical drive straight through. When the load gets excessive the converter comes out of lock-up and this seems like the box is changing down but it isn't. I had experience of an Allison CBLT4460 with this feature in a 30t dumptruck, and would be amazed if these Fodens were not similar.
So may not be the gear change, but the lock-up valve causing the harshness? I only mention this because the lock-up change is typically much more harsh than the smooth clutch pack gear changes.