I've had the same v4 address with ATT for over a year. It's technically possible for a dhcp server to deny a renew and have the client initiate a discover to be given a different IP but in practice that would break everyone's active connections each time so ISPs just let DHCP renew until you stop asking.

For something with the user population of a community game server you could also allow the full subnet and probably never run into a user collision.