" The InterNIC allocates addresses to ISPs based on the slow start procedure as detailed in the ISP guidelines. Please utilize the /19, once you have reassigned the /19 and SWIPped them you may request additional address space and if the InterNIC feels you are assigning them efficiently you may request additional address space."
Because of this policy ISP's should start getting allocations from the InterNIC as soon as possible. If you get allocations from another provider, you don't gain any history with the InterNIC. All of that history goes to the provider's allocations, not yours. When you outgrow a provider (and the provider's provider), and later go to the InterNIC, you start over again with 'slow start.'
Now, in various areas, I know that the NIC's procedures have changed over time. And I'm not talking about IP address space here, but wrt domain name allocations. But my experience was different recently. I found that the NIC (aka Kim) was willing to look at our allocation of a /19 from an old provider's space (in 205/8, so it was routable to Sprint). What caused problems for us is that we didn't SWIP it out as we allocated it, but instead SWIPped it out right before going to the NIC. I can't speak for any other cases, though. We went into fairly detailed justification for getting address space from the NIC, and ultimately got a 2-week turnaround time, which was much better than I had been lead to believe by others that it woud be. Along the way, it was made clear that the NIC wasn't interested in Sprint's filtering policies and would not make allocations based on it. Avi