If the "old log4j" is 1.2.x, you can update to reload4j (https://reload4j.qos.ch/), which is a fork of the latest log4j 1.2.x by its initial author; you don't have to update to log4j 2.x (which changed the API). And you might not even need to update, since "old log4j" (1.2.x) wasn't affected by that vulnerability in the first place (it has its own old vulnerabilities, but they're all in code which is usually not used by real world deployments).