I don't see any CSM.
This is no longer an issue for me, but as an academic exercise and in the spirit of feedback, I tried the conversion again with chgdtype.tbs and it was unsuccessful again.
As I previously reported, my conversion was successful and working.
Without making any BIOS changes, I restored a full disk image from when the disk was GPT. It worked fine.
Still with no BIOS changes, I went through the chgdtype.tbs conversion again. The result: a blank screen with a blinking cursor when trying to boot after the conversion was complete.
Again with no BIOS changes, I restored a full disk image of the previously successfully converted disk. Result: Fully working system.
There seems to be either something wrong with chgdtype.tbs or the way I'm using it.
All my conversions, both successful and unsuccessful have been GPT to MBR. I further converted the successful conversion from MBR to EMBR as part of BIBM installation.