Are the needed changes predictable enough that we could provide a script to convert the module? When we switched to using Maven, Ben made a shell script that converted a standard module layout to the new maven organization, added the pom, etc. That made it much easier for modules to migrate.
Could we back port support for slf4j? Or maybe make a separate module that injects slf4j support in oldest systems such that other modules could use it. This way, a module migrated to use slf4j could still run on older versions.