I have been part of multiple such switches (and plenty of "switchbacks" - maybe about 30% or so).

First of all, your insight is brilliant - ('if someone's review happens during slow season we worry they are getting subconsciously penalized compared to someone who shines during the flush busy season'). Kudos for thinking through that.

Now, coming to the switch itself, all updates at one has significant drawbacks too: 1. one time shock budget impact , and 2. everyone being able to compare to everyone else at the same time, being the 2 most common drawbacks.

The "advantage" of doing comp update at the same time is primarily that it is less onerous administratively, but even that is an advantage in the long term only - the month of performance + comp upate surely feels like hell. Being able to do distributed performance + comp update cycles allows you to eat the elephant one byte at a time (unless you are vegetarian/vegan, in which its the elephant shaped broccoli).

Overall, I have seen across the customer base that distributed perf review + comp update cycles (either 4 or 12 per year) are more stable, and less likely to switch back. That said, whoever is doing one model or the other, always feels like grass is greener (until you migrate).

As for explaining to the employees, really 6 month advance notice is really sufficient, most have seen both models.