Update: on my Voyager Legend the solution was to follow this other iFixit guide to gain access to the speaker, and remove three small screws that hold the boom to the rotary assembly. Only then can the boom come apart at its base. Plantronics Voyager Legend Earpiece Replacement

This set of instructions does not work. I am unable to separate my mic boom past the point indicated in the third photo of Step 1. After much careful prying, I reached a point where my only option is to apply a lot of force to pull the halves of the boom apart. Even then, the rotary section of the boom where it meets the main housing, refuses to separate. Something is amiss in these instructions, there is a step missing that should explain how to get the rotary housing opened up.