I had the same problem that after installing the public beta profile for Big Sur, I was only offered an upgrade to the latest beta from Catalina.
So I ran sudo as root via terminal and typed the commands there:
open terminal
sudo -i
enter root password
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil
Unenroll from the Beta Program
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll
Enroll in the DeveloperSeed Beta
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
Close and re-open System Preferences > Software Update
The new Beta should now show up
Steffen_Volgmann
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