How do you test changes to Jenkins plugins before deploying them?
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.