You sure can. Whether you want to keep things private with your team and then open them up to Reviewers, or you just made an error and it needs to be switched – you can do do easily. To get started, simply:
- Click the "Settings" in the Project
- Click the "Edit Settings"
- Select "External" (or "Internal" if you're going the other way)
- Save settings.
It's important to know, when projects are Internal, Reviewers will never see anything by default. When you have an External project, your Reviewers will gain access to certain things, but only has you explicitly share with them.
See it in action below.