If you want to test in the same Jira instance as the existing project, what I would recommend is to create a copy of the configuration scheme you are wanting to change (workflow, field config, screens, etc.) and applying it to a new project, then make your changes. It's probably good to have a "draft" copy where you make changes to test things first, then update the "live" schemes used in the existing project.
If you're on server, it's really best practice to have at least two Jira instances - one production instance and another development instance, and keep them in sync with each other by refreshing the data in the development instance every so often. Or, just have a development sandbox instance you can use as a playground for testing changes.
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