"if you change ISPs, as I did recently; on IPV4, the only thing"
Renumbering is a well known issue:
You can try to obtain a Provider-independent address space - but it's not for small users, I'm afraid.
With IPv6 is highly advisable to automate address assignments as much as possible, and use FQDNs as well. It's also better to "parametrize" addresses in rules, etc. to change them in a single place when needed.
This is another aspect that probably looked simple in 1996 with simpler, flatter, networks, few server and clients configured by SLAAC - not so much later as network complexity grew.