I followed the advices you gave in the Linksys EA8300 upgrade to 23.xx topic and successfully upgraded my router.
I flashed the factory.bin image, accepted the forced upgrade and chose to not preserve my settings.
The upgrade went fine. After the upgrade I had to re-setup my router from scratch - which wasn't a surprise as the settings was not preserved on purpose.
In my case that included setting up my PPPoE connection again with a user/pass that I manually backed up before the upgrade.
Other than the minor inconvenience of setting up a few static DHCP leases again, it was a pretty smooth sail - thanks for your help!