Downstream builds must keep working with optaplanner main (version 8).
The optaplanner repository must be migrated as a part of the upstream repository build command for the optaweb and quickstarts:
https://github.com/kiegroup/optaplanner/blob/main/.ci/buildchain-config.yaml#L30

Idea: push an environment variable STREAM_8 to the build chain. If this variable is defined (build originated from optaplanner/main), then migrate in the "upstream before" command. If not (the build originated from optaplanner/9.x), no extra steps are needed.