You already accepted it when you bought the CPU and it came with the base version. Your choices are to install a bugfix, or not. The FSF advocates for not installing the bugfix, or even informing users of the possibility of doing so. That is utterly illogical. There is no reason whatsoever to run broken proprietary software over fixed proprietary software.
No, that is a version you can't physically modify. The other version is one you can't modify because of it's license and signature verification. You are simply ignoring important differences, like saying an elevator is no different than a flight of stairs, they both get you up, and anyone who avoids elevators must be an idiot.
Your CPU comes with patchable proprietary software. Period.
There is some read-only memory that contains the base version. It is executed on boot. You can tell the cpu to run a different version by pointing it to a different version during the boot process (or later). You can't change the version in the read-only memory.
The way it works is that the microcode update gets loaded into some (smaller) RAM next to the ROM, and then patch registers are loaded which virtually rewrite some ROM locations to jump to the RAM (at least for the duration of this boot), in order to update the broken bits.
You are literally hot-patching the ROM with an overlay. You are not replacing it wholesale.
And either way, as I said, the underlying technical mechanism is completely irrelevant for the purposes of assessing user freedom. You're running proprietary code. You could be running a less buggy version of the same proprietary code. Choosing not to is silly and only hurts you. Trying to take that option away from users of your software is anti-freedom.
If the answer is "because then nothing could be certified or recommended" (which I expect it would be) then you need to stop marketing the certification as a recommendation for actual people to buy hardware and instead market it as a certification of whether the product is in line with their ideals.
> In other words, you can’t microcode update a CPU to add or substantially change capabilities.
There is CCC security presentation floating around where someone reversed engineered microcode before it was signed, and designed a backdoor into it, a remote code execution triggered by going to a specific webpage. That is a substantial capability that exists in todays microcode.