First of all, this method is only based on theory. This is not tested by myself yet. So you are free to read this article, but I not recommend to do so.
The method of downgrade is by flashing a dead phone. You can flash any firmware version on a dead phone. I read on the net that Nokia phones got security function to restrict older firmware installation. If you do so, the phone will dead. In my theory, once the phone is dead, now you can reflash the older firmware version to make your phone to live again.
Another step of getting dead phone is, you can flash the same firmware version, when the flashing process occur, quickly disconnect its usb cable. I don’t know how dead your phone if you do both methods. Either your phone screen (by pressing power key) will light up and off or it will not responding at all.
I don’t understand why peoples need to downgrade their phones. If they want to hack their phones, they can flash hacked firmware instead of installing original older firmware and hack it using HelloOX2. However, the hacked firmwares only available for s60v5 devices. So, I think this is why S60v3 owners need to downgrade their phones. But actually they can edit later firmware to put Patcher.ldd and patcher*.ldd into c:/sys/bin and install RompatcherLite after flashing that own hacked firmware.
So, this is the method to downgrade firmware based on theory only. I’ll take no responsibilities if you brick your phones by following this article.