upgrade job configuration with api?

[Makson Lee]

sometime, after upgrade a plugin, you just open a job configuration, do nothing, and then save it, you will find the job configuration has been updated to use the latest version configuration of plugin.

so, my question is, how to do it with api, i have tried to the following, but seems not working :-(

for (item in Jenkins.instance.getAllItems()) {
  item.save()
}