I ran wb3.1 on 3.0 roms for a long, long time. It was not until I ran into AmigaAMP which required 3.1 roms, that I softkicked the machine to 3.1.
I have been doing that ever since, and now it softkicks from 3.0->3.1, 3.1->3.9 so it is a few reboots when you first turn on your computer, but it is doable.
The only problem with the softkicking thing (I presume) is that if the computer crashes badly enough, it is simply crashed. Rebooting it will make it do absolutely nothing. I have had to turn it off and on a few times to get it to boot again.
Sincerely,
-Kenneth Straarup.