Random off topic question: could one theoretically (with infinite time and resources) write new microcode firmware for a modern processor that turns it into an armv8+ processor?
Not on these, since the decoder is hardwired for x86-shaped instructions (prefixes, etc). Some instructions are also hardwired to produce certain uops.
As others have pointed out the short answer is no. The longer answer is still no if you value execution performance at least a little bit.
However, maybe, there is a way. Back when we were researching microcode we found a talk [1] that ran multiple ISAs in parallel on the same processor using microcode. We never figured out how this worked, our best guess is either swapping microcode from RAM as needed or branching to an emulator in x86 code. If this was a K10 cpu, which might be a bit old at the time of the talk, then there is no way you could fit an ARM interpreter into the update. You had, iirc, 32 triads of 3 operations each. Maybe, just maybe, you could fit a bytecode interpreter that then executes the actual ISA emulator. However you would need to hook every instruction, or at least trap on each instruction fetch and hook the appropriate handling routine and both sound very complicated.
If your infinite resources include manufacturing new silicon with the proper fast path and microcode decoder, then yes, but note that x86 and ARM have different memory models. Also at that point you just have a very expensive, very inefficient ARM processor.
Without more in depth knowledge here my guess would be yes, if you can fit an Emulator for armv8 into the size available for microcode. The instruction the cpu runs vs those that are emulated via microcode are already pretty extensive, running essentially an ARM Emulator on Top of it should in theory not make too much of a difference since you are essentially running an x86 Emulator on whatever the ryzen instruction set really is.
in the introduction they explain that it's not possible
"The first question everyone has about microcode updates is something like "So can I execute ARM64 code natively on my Athlon?" It's a fun idea, but now we know that a microcode patch doesn't work like that -- so the answer is no, sorry!"