In 2023 I created a N-of-M 20-word Shamir seed backup.
I understand it is identical to what is now called Multi-share Backup aka SLIP39?
How can I now create a master-single-share-SLIP39 20-word seed?
So basically “downgrade”. Got my reasons to have both the Multi-share and the Single-share backups each for specific scenario for the same wallet.
Trezor Suite?. Trezorctl?
That is not currently possible for backups created before June 2024.
There was a tweak to the backup format, making it possible to create a different backup type out of the existing one. Your backup is from 2023, before the tweak existed.
Technically, you could create another share by loading your backup into a PC and use a Python program to regenerate another 1-of-1 share with the same SLIP-39 identifier. The resulting share would have first two words the same as your current one, words 3 and 4 would be academic academic, and the rest would encode your wallet.
Currently, however, there is no user-friendly tool for doing this process.