Hello Dominique
The idea behind my mentioning Prefix Delegation was the fact that using DHCPv6, the ISP could indeed change the prefix that is provided to each individual customer whenever they would like. They can do it manually, but they can also set a valid-lifetime and a preferred-lifetime. The command would be issued like so:
prefix-delegation pool poolname [lifetime valid-lifetime preferred-lifetime]
Notice this context sensitive help:
ISP(config-dhcpv6)#prefix-delegation pool GLOBAL_POOL lifetime ?
<60-4294967295> Valid lifetime (seconds)
at Expire prefix at a specific time/date
infinite Infinite valid lifetime
ISP(config-dhcpv6)#prefix-delegation pool GLOBAL_POOL lifetime 600 ?
<60-4294967295> Preferred lifetime (seconds)
infinite Infinite preferred lifetime
You can specify the number of seconds, or you can specify a fixed duration of time for hosts to remember router advertisements using the at keyword, where you can actually input a specific date.
You can find out more about this command and all of its features at the following Command Line Reference link:
To find out the difference between preferred and valid status of prefixes, take a look at the following RFC4862 link. Although it is speaking about IPv6 addresses, this is valid for general prefixes as well.
I hope this has been helpful!
Laz