Yes, you can downgrade. I just did with my three IPC-HFW4300S's which were on 2.420.0008.0.R, build : 2015-07-10
I downgraded them to 2.420.0000.10.R, build : 2015-04-24
The version I downgraded to appears newer in file version, but older in date. Regardless, I "upgraded" the firmware using the System section of the web interface and everything was back to the way I wanted it, with the exception of the FTP settings being wiped, but that is normal with the firmware change.
When I upgraded to 2015-07-10, I realized there was no IVS, so I immediately re-upgraded (downgraded).
I won't upgrade again unless the version has IVS. It nearly eliminates false-positives.
Thanks Euly...the firmware 2.420.0000.10.R, build : 2015-04-24 with the IVS works great with my IPC-HDW4300s v2! Great heads up.
