migrating your whatsapp message history

i've had to change/use a different phone a few times since becoming rather dependent on whatsapp, "a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS." in the past, i've considered it "too hard" to move the message history with a new installation. this time round, in preparing to send my currently primary phone in for repairs, i tried to figure it out on my own. it didn't work... the correct solution is actually pretty easy but dependent on a specific order of steps. nobody will keep using the same phone forever, so this will come in handy sooner or later.