POSIX support is easy if you use the new generic getaddrinfo and getnameinfo. Code needs to be ported from the old way which hardcoded IPv4 addresses (AF_INET). A properly written program will support both IPv4 and IPv6 and will use the right one based on network interfaces and DNS.
In practice, IPv6 is no different than IPv4 with NAT for tracking people. With IPv4, your machine address is hidden by NAT behind your public IP address. They can track it to your house but not your computer.
By default IPv6 uses the MAC address in composing the IPv6 address which ties it to the machine. But this is recognized as a problem and there are ways of creating unique random addresses. Also, there is enough address space that you can change your address frequently. Every day or even every destination. They can still track the prefix to your house.
If they had just hashed the MAC address, it would be harder to predict and not obvious it came from the MAC address. Hashing it with a secret key (but shared key) would probably have been enough security. They would have a problem if the key was compromised but it could be model or firmware version specific.
Break out the Fair Debt Collection Practices Act and get them to stop. Answer the phone and get their name and address and all the info about the debt. Mail them a written request for verification of the debt. After that, all contact needs to be by mail. They are already doing things they shouldn't like calling late at night. They are supposed to have mailed you a description of the debt. Keep a log of all of the calls and violations. Send them to the FTC if they don't stop. You can even sue them and get $1000 in damages for violations.
"Most people would like to be delivered from temptation but would like it to keep in touch." -- Robert Orben