Well, their advice for changing strings is for only that. It's best to, when you can, keep the tension put on the neck by the strings as even as possible when, say, switching old strings out for new ones. so if you are restringing, you'd do it one at a time. But to put a new pickup in, your guitar strings are going to be in the way of everything, so you'll have to remove them. It's not a big deal though, it just means you will need to check the bow of the neck after the pickup switch and re-stringing process is over to see if a truss rod adjustment is in order. If you are unsure of yourself, especially when it comes to the truss rod or wiring, it might be a good idea to consult a good luthier, at least for advice.
