This explanation rocks!!! Its valuable piece of information I needed. Thank You so much. I will wait till summer 2017 with all 2200,3300, and 4300 switches since the environment is a school, so summer would be perfect time for upgrade. I have to prepare game plan for the upgrade and prepare myself since this will be my first Juniper software upgrade. Probbaly in summer 2017 I will start from 2200 (small IDFs) then 3300 virtual chasis (medium IDFs) and finally 4300 virtual chasis of main MDF.
I will come back to this topic in summer 2017. Thank You so much again for the information.