Part 15 devices are designed to be installed and used by the general public. With this in mind, the commission wants them to be as "idiot-proof" as possible. They have severe limitations on what you can do with this gear. For instance, the rules state that:
15.203 - An intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device.
A bit further in, the same sentiment is repeated:
15.204(c) - Only the antenna with which an intentional radiator is authorized may be used with the intentional radiator.
The basics of certification can be found in FCC 2.901 through 2.1093. The requirement for Part 15 devices can be found in 15.201.
Equipment can be certified in two ways: as a component or system. Component certification applies to equipment such as transmitters, amplifiers, or antennas. All can be mixed and matched with each other. However, if you have various equipment certified as a system, the parts of that system can't be used with other equipment. See 15.203 and 15.204:
15.204(b) - A transmission system consisting of an intentional radiator, an external radio frequency power amplifier, and an antenna, may be authorized, marketed and used under this part. However, when a transmission system is authorized as a system, it must always be marketed as a complete system and must always be used in the configuration in which it was authorized. An external radio frequency power amplifier shall be marketed only in the system configuration with which the amplifier is authorized and shall not be marketed as a separate product. [Boldface added by author for emphasis]
In other words, you can't take an AP that is certified as a system and attach an antenna that isn't a part of its certification.
You can, however, recertify equipment. If you go out and purchase gear on the street, there isn't anything to stop you from reselling this gear and recertifying it at the time of sale. There seems to be some discussion as to whether you need approval from the manufacturer for recertification, but I talked to one communications law attorney who said that approval is not needed.
Certification is an involved process and can be costly. You should contract with many of the consultants in this field for guidance.
Experimental licenses are used for temporary experimentation, while STAs are issued for spectrum use in emergencies or other critical situations. In cases where time constraints make it impossible to go through the traditional paperwork process imposed by the FCC, an STA is granted. STAs are limited to six months of authorization, while experimental licenses can last for up to two years. STAs have a lower priority for interference than experimental licenses, but since they are issued to Part 15 devices, this doesn't matter much.
STAs and experimental licenses can be used only for very specific purposes, such as legitimate educational research. For instance, they cannot be used to "determine customer acceptance of a product" or for "marketing strategy."
For more information, visit the FCC web pages on STAs and experimental licenses at http://www.fcc.gov/oet/info/filing/elb/.