IPv6 address configuration

I'm about to upgrade my network connection and my ISP tells me I'm going
to have to have new static addresses because I'm moving from a VDSL to
fibre infrastructure. This isn't too much of a problem on IPv4 because
NAT means it's just a single change at the router (+ DNS changes of
course), but is a nuisance on IPv6 because I'm going to have to renumber
every machine (and most of them have multiple addresses because I use an
IPv6 address per service as well as one for the machine).

What would make my life easier is if ifconfig had a way to set IPv6
addresses where I only specified the lower 64 bits and it got the /64
prefix from router solicitation, similar to the way SLACC addresses are
formed but not using the hardware address. Is this possible because I've
not found a way to do it?

[Yes, I know about NPTv6 and will use it as a last resort, but I prefer
internal and external addresses to be the same so I don't have to do
split horizon DNS for IPv6.]

-- 
All network cabling aspires to the condition of macramé.