[EricH]

If it involves opening the equipment, see the following:

<R60> The control system is designed to allow wireless control of the ROBOTS. The Classmate PC, FirstTouch I/O module, cRIO-FRC, speed controllers, relay modules, wireless bridge, batteries, and battery charger shall not be tampered with, modified, or adjusted in any way (tampering includes drilling, cutting, machining, gluing, rewiring, disassembling, etc.), with the following exceptions:
[...]
H. Wires, cables, and signal lines may be connected via the standard connection points provided on the devices.
I. Fasteners may be used to attach the device to the OPERATOR CONSOLE or ROBOT.

(bold is my emphasis)

I'd say that's most likely a no to changing the antenna. Ask in Q&A, though; you never know.