i would say to switch the firmware is save as long you never accidently flash a firmware 3.55+
the time to switch depends on how you switch.
if you are able to switch firmware over xmb via debug settings with a qa flagged console arround 3 minutes.
if you switch firmware over recovery mode arround 7 minutes.
unfortunately sometimes for an unknown reason a switch is only possible over revovery mode.
i hope the reason why mentioned switch via xmb sometimes give the error "same firmware is already installed" will be uncovered.
CECHA12 160GB NANDway Double Teensy2++, CECHA00 750GB, Both Consoles europeanized, REBUG 4.46.1 REX+GNU Linux Red Ribbon RC7