When I still worked in the ISP world, the startup I worked for started off with PA space, and then grew into PI space, and handed the PA space back to their upstreams as it was vacated. I had no problems getting subsequent PI blocks because our documentation was in order.
The documentation isn't the pain. The renumbering is, *especially* if you're running a service provider network: 'Dear dedicated server customer, we're taking away your IPs, please don't be angry with us even though it will cost you untold hours of work to hunt down all the tiny implications of renumbering. Never mind the lost business it might cause if you miss something.' 'Dear internet access user who happens to run a bunch of IPSEC tunnels: Have fun fixing all your tunnels! Don't worry, we'll figure out an off-hours time that works for everyone, and that makes all the pain go away, right? You won't harbor any resentment, right?' (Wow, that comes off more bitter than I expected...) Oh well... Since new IPv4 allocations are fast approaching the same scarcity as unobtanium, I guess it's too late to worry about it now. Anyways, apparently IPv6 fixes all of this, or something. Nathan