Is it possible to secure applications using old adapters (say 1.6.1) with a keycloak server of more recent version (say 1.9.0) ?

The question boils down to what is the proper upgrade policy in a keycloak secured system with many applications provided by different customers. If an application with an old adapter does not work with a newer keycloak server then it seems all (both keycloak and applications) should be upgraded in a single step.