Buster to Bullseye upgrade followed by a downgrade back to Buster I want to switch from Debian Buster to Debian Bullseye. After doing this, can I return all Bullseye packages to Buster? (Unupgrade packages)

Technically, you can, but downgrades aren’t supported and we don’t test them. The downgrade might work fine, but it might not, and Bullseye is now sufficiently far ahead of Buster that I wouldn’t try it.