You definitely can change domains added as a project anytime. It’s entirely up to you.
What you cannot do is add two projects with the same domain.
To change the site in a project, if you already used all your projects, you will first have to remove one. You can learn how to remove a project here.
Once you have removed the project, you’ll then be able to add a new website you would like. Notice that removing a project means you will lose all the historical data collected while that website was on your Dashboard.
In the case that you need to keep all your existing projects, reached the project limits, and need one more extra, you can check the new add-ons option. With this add-on, you can exceed your project limits by paying a specific monthly fee for each one.
You can find more information about add-ons in the article How do Add-ons work?
To learn the project limits of your plan, visit the article Can I add more projects to my Dashboard?