this is not 100% correct:-) If you own an Amiga 4000 equiped with an Commodore A3640 Board you can "softkick" an Rom3.1-file into your RAM and run OS 3.9 without any problem on an Amiga with 3.0 ROMs:-)
I call that trouble. I've softbooted 3.0 1200s with cheap '030 accelerators to run 3.9, sure it works, it's just not as nice as having real 3.1 ROMs.