Moving a home user's service from a real IPv4 address to a NAT IPv4 address is a problem if the user is an EFF member, extreme gamer, Bittorrent user, or some kind of nerd.
As a big ISP, you could re-brand the change as an upgrade to "Family Friendly" or "High-Security Firewalled" service. Start out offering it as a paid upgrade, then make it "free for a limited time", then the default for new users. If it breaks some network service, just say that the other end doesn't like Families or Security.
This is so much easier than IPv6 that I don't see why the ISPs won't do this instead. (In Karachi, Pakistan, an ISP will sell you 100Mbps Ethernet to your neighbors, decent backhaul, and NAT for $8/mo. This works fine for Skype, Facebook, and YouTube.)