I pushed back on that requirement due to cost and availability issues, but went ahead and installed the polarity switch on our CP antennas. With certainty I can tell you that at random times it made a huge difference in downlink signal level.
There was discussion on the ARISS OPs mailing list as to what to recommend to schools and what would give best "bang for buck" I am still not 100% convinced that a RHCP/LHCP switch is indeed necessary for ARISS contacts. The only way to confirm would be one station with both RHCP and LHCP both receiving at the exact same time and logging or recording the results. Most/all stations that have said the switch helped have not compared in real time not switching. Over time the ISS signal can vary wildly, in particular the solar panels can totally block the signal completely from time to time. There is a couple of stations with dual VHF circular systems (mostly ARISS tele-bridge ground stations) and I have asked them to help. Until we have real world comparisons at least in the case of the ISS, I am not sure a switch is needed. Can someone do tests to check?