802.3at is backward compliant with 802.3af, which means that 802.3at equipment will be able to send power to 802.3af devices. 802.3at has a higher power limit, which means more wires can carry more current. If the device does not ask for more than the 802.3af specification is configured to provide, then the 802.3at circuit will not be activated.
The MR11, MR12, MR14 and MR16 are 802.3at (and af) compliant. These APs will send an “802.3at style” handshake, but will draw 802.3af power levels.
The MR58 is 802.3af compliant. It does not send the new handshake, but draws at a maximum rate needed for 802.3af.
802.3af and 802.3at standards are generally cross-compatible.