Can I change the risk workflow?

Yes. The Risk Register add-on is generally tolerant of changes that you might make to the risk workflow. If you remove statuses from the original workflow configuration, or remove transitions between those statuses, then some of the add-on's automatic workflow progression will be disabled, but in other respects everything will work just fine.

If you get really stuck, visit the Risk Register settings page where you will find a 'Reset risk workflow' button; clicking that button will return the risk workflow to its original configuration.