EFI Secure boot vs Legacy boot.
If you have not enabled EFI boot (and very few have yet, it is still kind of new), that bullet point does not apply.
If in the future you decide to rebuild the server using EFI secure Boot, then you would need to be careful about trying to flash the BIOS down to a lower level in the future. It is a corner case, but need to be covered in the release notes.
The bigger issue for most customers is the NVRAM table update.
Any customized BIOS setting you may have already set will be restored to the defaults and need to be set again
(This does not impact BMC settings in BIOS set-up and sometimes it is difficult to tell if a setting if BIOS or BMC. )
(Pretty much anything in the server management tab is BMC )
If in doubt, i would download the SYSCFG tool and use it to save your current BIOS setting to an file called syscfg.ini. (syscfg /save)
The rename it Old.ini.
After flashing the new BIOS run syscfg /save again to save a syscfg.ini of the new BIOS settings.
You can open the .ini files in a text viewer (notepad or beyondcompare work well) and see what values need to be changed back.
Unfortunately, syscfg will not restore the setting for you across the BIOS level change so you need to do it manually.
The STARTUP.nsh script file takes care of loading the back-up bios automatically. Don't panic after the first reboot. The back-up BIOS updates in the background on the reboot and it will cause the screen to stay blank for about 3 minutes while the update is occurring.
Last piece on Secure Boot.
If you decide to enable EFI secure boot some time in the future, you would need to install a EFI Boot OS. Your legacy loaded OS would not be recognize and only devices formatted to EFI would be viable pre-OS. a FAT32 USB keys would not be, but you can format a USB key to EFI if desired.
If you enable the uefi secure boot, it will still boot to any uefi formatted media.
I have not tried to disabling it after you get the OS loaded when i wanted to keep the OS.
Its kind of like messing with a RAID controller after making your RAID. OK if you know what you are doing, but best avoided.
If you do disable, any media will be recognized again,
The only issue is if you try to down grade the BIOS for some reason after you enabled the secure boot
Never figured out any reason a person would down grade unless they just like known bugs.