If you have a vanity domain mapped to one of your ScreenSteps sites you can change it at anytime you would like.

You just need to be aware that any incoming links that point to the old domain will be broken. You can't set up a redirect in ScreenSteps for links from the old domain as ScreenSteps won't know about the old domain anymore. 

You would need to contact your DNS provider to see if you can redirect the old domain to the new domain.