You should also include a number of additional cost of reassignment, including growth of routing table size.
I'm sorry to inform you that apparently your ISP, your workplace and your university are all either lazy, lying, or both. Implementing IPv6 takes, maybe, a few weeks if you have many servers and a large internal network. If all you have is a webserver which you want to be reachable by IPv6-only customers, an interim solution (tunnel) can be implemented in a few minutes.
Sure - from your router's subnet. The route to that address still goes to that router. There's no BGP update to propagate, no routing table entry to add to every router on the internet.
I mean, why not assign every IP address individually? That way we'd be fine for up to 2^32 internet-connected devices. Every router would just have to store all 2^32 routes in memory and send out a BGP message whenever anyone rebooted. Can't see any problem with that.
I get assigned a new ipv4 IP by DHCP every time I reboot my computer. And you should hear the infrastructure people at my ISP and workplace and university squeal when you ask them about ipv6 - far too much work, they say, no plans on the horizon.