Is setting CIVIL_UPGRADE_PROMPT_OFF=0 the same as %undef it?

I want to make sure our users are always told when a file they open is going to be upgraded to a new schema, but some of our client workspaces (and occasionally workset.cfg files outside of our control) turn off the prompts by setting CIVIL_UPGRADE_PROMPT_OFF=1

I'd like to prevent this variable from being set but you can't %lock an undefined variable to prevent from being set. Instead, if I set CIVIL_UPGRADE_PROMPT_OFF=0 and lock that, will it have the same effect as being undefined?