Hi Jeen,
thanks for the initiative.
We are widely using the constants in our code base (especially RDF, RDFS, and OWL) and have followed the RDF4J naming scheme for our own vocabularies.
From my side: I am also very much in favor of keeping the class names upper case (makes it also easy to read that these are vocabulary constants). Regarding the field names I do not have a strong opinion - likely switching to camel case is the better option (to better match the local name of the property / class).
When changing this I would be in favor of having one release, where both variants (the existing one and a potentially different one) are present - makes it a bite more easy to migrate. Note that for me it's fine to do this in a 4.x release and do the breaking part in 5.x.
For the repository configuration constants: looking really forward to have this cleaned up. The namespace proposal you have makes sense - I'll also shortly comment on the PR. For the change in repository configurations I would however vote for having the backwards compatibility over one major release - as this does not only affect code, but also serialized repository configurations.
Best,
Andreas
