47 CFR § 73.761 - Modification of transmission systems.
Specific authority, upon filing formal application (FCC Form 309) therefor, is required for any of the following changes:
(a) Change involving an increase or decrease in the power rating of the transmitters.
(b) A replacement of the transmitters as a whole.
(c) Change in the location of the transmitting antenna.
(d) Change in the power delivered to the antenna.
(e) Change in frequency control and/or modulation system.
(f) Change in direction or gain of antenna system.