1. use backup utility to backup each ps3. The only thing it doesn't backup is the trophies.
2. use data transfer to transfer from 320 to 120. assuming there is enough space
3. downgrade the 320, hdd will be formatted.
4. use backup utility to restore the backup for each console
This is assuming the stuff on the 320 will fit on the 120.
Or I have a better idea,
Downgrade the 320. ;P
If you do not even know if it is downgradable yet, I would worry about it after you find out.