B350M-Mortar won't boot to Windows after CPU and BIOS update

[martij152002d6]

I wanted to upgrade my old PC from a Ryzen 1600 to a Ryzen 5700X3D. According to the web, the B350M-Mortar supports that CPU.
I put in the new CPU without updating the BIOS first (because I'm an idiot) and run into a million problems. Reset CMOS, go back to old CPU, get everything working again, update the BIOS to the latest version at https://www.msi.com/Motherboard/B350M-MORTAR/support#bios (which is labeled "beta", but everything after 2019 in labeled beta).
I boot to BIOS, where the UI looks completely different, totally unlike the familiar red-black scheme I had and see everywhere online:

Other than that surprising change, everything seems fine, put in the new CPU, still boots fine, set a max temp for the CPU because X3D CPUs are apparently sensitive about that, and boot to Windows...
...only to arrive back at the BIOS.
So I check the boot order (see above), and I see it's all set to UEFI, not Legacy. And I can't change that. What the hell?
Should I not have gotten a beta version? I try to downgrade to the latest non-beta version, but the BIOS won't let me, because it wants me to have the best experience. Reader, I'm not having the best experience here.
So what's up with this version being hard set to UEFI? I strongly suspect the old version supported Legacy, and I strongly suspect that's what I've been using.
Also, is the new UI cause for concern?
How do I get out of this?

 

[marcin.ozdzinsk153802d5]

You can install new Windows or convert it to UEFI using recovery from USB:

Redirecting

 

[martij152002d6]

It turns out that under Advanced -> Windows OS Configuration -> BIOS CMS/UEFI Mode, I can still set it to CMS. Did that, booted to Windows, ran mbr2gpt, which converted the ssd to GPT boot system. Only it gave a warning that WinRE is broken, and I've been unable to fix that.
Set the BIOS back to UEFI and now it boots fine. My only worry is that WinRE is broken and none of the advice I can find online works. But that's got nothing to do with my motherboard+BIOS anymore.

 

[marcin.ozdzinsk153802d5]

Tried this ?

Can't enable recovery partition after conversion to GPT - Microsoft Q&A

Converted to gpt (UEFI), got an error "failed to update reagent.xml, please try to manually disable and enable winre". Can't enable it, first because it couldn't find files, then I extracted winre.wim out of an installation .iso and put it in…

learn.microsoft.com