We just went into this subject with another user a day or so ago.
There are AP models and iAP models.
An AP is an AP is an AP (controller only)
An iAP is either - there's a conversion method.
A user must purchase the iAP if both possibilities might be needed.
http://community.arubanetworks.com/t5/Aruba-Instant/How-to-make-104-into-Instant-104/td-p/68222