It is not a general rule that you can't go back to the previous firmware version. Sometimes you have to downgrade to even earlier version just to be able to upgrade to the version you want.
I used to have QSI-SDW-042 (it was replaced later with QSI-SDW-082 by service) and after a firmware upgrade form DX70 to newer from NU site I still had problem burning certain brands of quality DVD media. The problem was getting worse as newer media came to market replacing compatibile old ones. Before returnig this drive I wanted to check if the previous firmware DX70 would do better. The only way to do this was to downgrade the firmware below DX70 (as I remember it was DX35 and the drive description changed from 042 to 041) and then upgrade to DX70. After that everything was back to normal, yet still not satisfactory.
When I got my new sdw-082 and noticed that it's also very media sensitive so I upgraded firmware to LX44 (the one from aopen site). I haven't noticed much improvement so I tried flashing with different firmware and with LX43 from aopen site there waren't any problems beside the notification, that the version is older than the current one. The other firmware including any form the NU site and the ones for phillips couldn't be loaded as it returned error before flashing.