How do you test changes to Jenkins plugins before deploying them? If you ever been bitten by a plugin upgrade that broke some functionality you must have given some thought about this problem: What should be the Jenkins plugins upgrade policy? How do you test changes before deploying them?

Has anyone gone as far as having a test instance running dummy jobs to test the new versions or do you just pray that upgrading the versions won't break anything?
According to the policies of the company where I work we have dev, preprod and prod environments (on some service dev can be missing). And the path of new version preprod->tests->validation->prod.

In our case jobs in preprod are heavy and complex enough to be sure we do not need to pray when implement in prod :)

Note: We use svn to maintain and deliver configuration. We do not make changes inplace.