IPv6 uses provider aggregated IP addresses. In the documentation you can read, how easy it is to do address renumbering.

You can specify a general prefix on a device. If you need to change the address, you have to change the global prefix only, and all addresses got the new prefix on this device.

On cisco devices it is:

ipv6 general-prefix PROD 2001:DB8:1234::/48
!
interface Vlan1
 description Management Network
 ipv6 address PROD ::1:0:0:0:2E1/64
 ipv6 enable
!
interface Vlan2
 description Management Network
 ipv6 address PROD ::2:0:0:0:FF/64
 ipv6 enable
!

Now you change the address of the generel-prefix PROD and all your IP addresses on this device are changed.

Cisco does support PPPoE with IPv6, so if your provider does support IPv6, you can do a native connection. If not, you can establish a Tunnel to SIXXS or another IPv6 provider.

ipv6 unicast-routing
interface tunnel0
  description IPv6 uplink to SixXS
  no ip address
  ipv6 enable
  ipv6 nd suppress-ra (<12.4)
  ipv6 nd ra suppress (>=12.4)
  ipv6 address [Your IPv6 Endpoint]/[Prefix Length]
  ipv6 mtu 1280 (or other MTU value)
  tunnel source [Your IPv4 Endpoint]
  tunnel destination [PoP IPv4 Endpoint]
  tunnel mode ipv6ip
!
ipv6 route 2000::/3 [PoP IPv6 Endpoint]