(I am NOT changing from the 3.1 40.68 commodore roms) Thank you
Technically speaking, you don't have to open up and change ROMs. You can Softkick with a ROM image.
If you have already paid for a 3.9 ROM, this is a legal option. Unfortunately, somebody appears to have wiped softkickers from Aminet. MMU.Library utilities are only available there now. 32 bit memory only, must have MMU capable CPU fitted.
Come to think of it, Aminet seems to have been carefully pruned over the years. Or maybe it's just a temporary glitch.
(I don't think that last idea is at all feasible, judging from what I have observed. Looks more like a subtle Amiga ROM takeover bid).