This is one of those blog posts where I (ab)use blogs.perl.org to document some notes, primarily for myself ...
Server A : is the old "Master"
Server B : is the new "Master" (a rebuild of A) and for a transition period is replicating from A until it is promoted to the main Master and A can be switched off.
Server C : currently replicates from A, however we want to instead move it to replicate from B in advance of replacing A with B.