Reconfiguring a launchd job
You can tell launchd to remove it from its cache and reread from disk. From the man page:

 uncache service-name
          This subcommand instructs launchd to bypass its service cache
          for the named service and instead read the service's configu-
          ration file directly from disk.  launchd maintains an in-mem-
          ory cache of XPC service configuration files to minimize the
          disk I/O. This subcommand will remove a cached entry so that
          developers may more rapidly iterate on a service's configura-
          tion. It should not ever be used as part of production work-
          flow.