Yes, I'd simply add a new AG and make sure it is set to allow agent routing, which is the default option for AG installation. Once deployed, all agents will get the updated list of AGs and you should be able to remove the one you want to decommission. Just make sure all agents will have a route to the new AG, if they cannot reach it due to firewalls or routings restrictions, it won't work of course.
Are you making use of network zones at all? Is the AG you want to remove, part of an NZ?