In this answer I discuss that the Voyager spacecraft can be commanded to use either high or low power to transmit to Earth on X-band.
Question: Are the Voyager spacecrafts' X-band Traveling Wave Tube Amplifiers (TWTAs) currently set to high or low transmit power? How often were they changed?
From DESCANSO Design and Performance Summary Series Article 4; Voyager Telecommunications by Roger Ludwig and Jim Taylor:
3.3.4 X-Band Exciters
The X-band exciter converts the frequency at the output of the S-band exciter to X-band to drive the X-band TWTA. Comparable to the S-band exciter function, the X-band exciter phase modulates the RF signal with the composite telemetry signal from the TMU and, if the X-band ranging channel is on, the ranging signal detected by the receiver.
3.3.5 X-Band Power Amplifiers
Only one X-TWTA can be powered at a time. Further, a control input from the CCS ensures that the X-TWTA is powered off when the X-exciter is off. As is the case for S-band, whether powered on or not, the X-TWTA power level is selected to either of two levels4 by CCS control input.
In October 1987, the Voyager 1 X-TWTA-2 failed, as annotated in Figure 3-1, legend item 3. The primary downlink was switched to X-TWTA-1. In November 1998, Voyager 2 switched from X-TWTA-2 to X-TWTA-1 .5 For both spacecraft, legend item 1 in the figure flags the changes from X-TWTA-2’s right hand circular polarized downlink to X-TWTA-1’s left hand circular polarized downlink.6
4The low-power and high-power RF levels to the HGA for the X-TWTA are 12 W and 18 W.
5The switch to the backup X-TWTA is in status report http://vraptor.jpl.nasa.gov/voyager/pressrel/vg981117.html
6The relationship between X-TWTA selection and the resultant polarization of the X-band downlink is described in the SXA section that follows.
Foot note #4 indicates that there is at least one method to decrease the transmit power of Voyager built into the system. A command from the ground can instruct the X-band High Gain Antenna (HGA) Exciters to change between 18 W and 12 W of transmit power delivered to the antenna. But what does that mean in terms of electrical power?
Figure 3.1 from Article 4; Voyager Telecommunications (click for larger), discussed further in the question Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end & IF?