Can I Replace The Custom Domain Anytime?
Definitely. If you are running out of the custom domain quota or want to change to a new domain name, just delete the existing one and simply add a new one.
What happens to the existing funnels with the old domain?
It will automatically switch back to the default systeme.io domain. Once you have added your desired new domain, just go to Funnel settings to change the domain back to your own domain.