We are in the process of looking for a new colocation provider and we currently use our Provider's IP Space for our servers. So we will have the process of DNS changes and renumbering, etc.
Our current provider does not allow for PI space so with them it was not an option
What is involved in using Provider Independent IPs ?
We are a small company and probably can't get ARIN to give us our own space (or can we?). We do have a parent company with it's own space that they could possibly give us a piece of. We can just take a piece of a class C or does it need to be a full class C?
How does PI address space work and how does it get routed to your colocator that may be running 3-7 Internet Providers for redundancy?
Are there any drawbacks to using PI space over the providers address space?
To qualify for an ARIN allocation, you would need to be multi-homed with at least a /21 fully utilized, or single-homed with at least a /20 utilized. As you said, that option may not be available to you.
If you use space belonging to your parent company, you will need at least a /24. Some networks these days, notably Verio, are aggressively filtering long advertisements in "new" address space, so you may find that even if you borrow a /24, if it's in 66/8 for example, not everyone may hear your advertisement. If that is the case you can hound folks to add you as an exception to their filters, or you can arrange a tunnel from your parent company. Neither of those are great long-term choices.
The above is a perfect illustration of the disadvantages of provider independent space. When using provider assigned space, they are very motivated to ensure that the space they re-assign to customers is globally reachable. That is often not the case with provider independent / portable address space.
As to your question of, "how does it get routed," your provider will need to advertise the space you bring with you via their BGP sessions. This is very routine, and any provider whose technical staff don't immediately know what you need in this respect is a poor choice. Beware of clueless "engineers!" :-)
Actually there is a proposal waiting approval from the ARIN trustee's that would make the minimum allocation a /22. They have already put the new fee structure online but haven't come right out and posted the /22 info publically yet.
If you want to wait for a month or two this could be an option.
Both the multi-homed /24 allocation policy, as well as the proposal to lengthen the minimum allocation size, have been in the works for years with no signs of implementation. The members want it and have voted in favor of it at ARIN meetings several times, however there seems to be little interest among the ARIN staff in getting it done. Good luck getting an allocation under those policies.