Depending on the version of Jive you are running, you might have access to a UAt environment. If so, you can request a PROD -> UAT data refresh to be run on your instance which would sync over the data in your PROD instance to your UAT instance for you to run through tests with. Alternatively, if you are in a Jive Cloud instance you might have access to a Private Preview instance ( FAQ - Cloud Private Preview Program ) which can also be refreshed with PROD data.
However, the last part of your inquiry:
3. After we finish making changes, push a button to replace everything from current space with what's in the sandbox space.
Is not possible in the out of the box options as only PROD -> UAT data refreshes are available. That being said, if you are interested in developing a custom solution for your instance that would allow you to migrate content from your UAT instance back into your PROD instance, I would suggest setting up an engagement with our Professional Services team who can assist in determining the scope for a custom feature in your instance. To request an engagement please either reach out to your account manager or submit a case in your customer group ( MyJive Secret Support Group Administration FAQ ).