your external IPs might change, such as when moving between ISPs

This is true

You would NAT a hosts external address to its internal address.

This is usually not true.

If you're worried about your external IP changing (like if you're hosting a server on it), you'd solve it the same way you solve it with IPv4: Using dynamic DNS. The main difference is that you run the DDNS client on the computer rather than the router. If there's multiple systems you want to be able to access externally, you'd habe multiple DDNS hostnames.