So I had to do this on their T6 line. If Feniex kept up the interchangeable heads (and it looks like they did) it should swap out on the fusion line. I only see 1 problem.
Dumb head vs smart...smart heads might have delay when activating. Feniex used smart heads in their 1x and 2x and dumb ones in some other products (roof bars). As long as the stick has smart heads already then it should be direct swap.