I recently added another switch into a stack of five 7650s that were doing 40Gig on the stacking uplinks. By default, the 7650 will want to do 100Gig. In this case, I needed to change the speed from 100G to 40G. Below is how I did it.
ICX7650-48ZP Switch(config)#rear-module stack-40g
ICX-7650 rear module setting changes requires a reload to take effect!!!
The rear module operates in stacking mode with 40Gbps speed after reload.