Without sounding like Captain Hindsight, the data sheet identifies the different part numbers and thir role. There are a HOST of reasons why we have separate parts. Some customers (think high security like financial, government, etc) where they don't ever want the APs being able to work autonomously. In those cases they buy AP-325s. For others where they want autonomous, or don't have security restrictions/policies against it, can buy IAP-325s. Because the IAPs have to come up as Instant fisrt, they can be converted, but if you are a high security, or government customer, where they can't allow an IAP to come up on it's own, the IAPs are not an option.
Best bet, to be perfectly frank, re-sell the AP-325 for hopefully as much as you paid (maybe for even more if you're great at Marketing), and then buy an IAP-325. Or look for a 7xxx based controller (the 7005 will be lowest cost) to run the 325. You get a small bit more features with the controller-based solutions if that interests you.