Disclaimer: I am not intending on doing this. It is just a question I thought was fascinating and might be useful to other writers.
Here's the scenario. You're writing a series of novels. After the first book, you decide to change to a new protagonist. The reason isn't important. What's important is that if you've developed the protagonist correctly, the reader is invested in him. He wants him to win. Now he must suddenly shift to a new protagonist.
This presents a problem. The reader will want to stay with the old protagonist. That's the one he likes. He spent a whole novel with that person, learning deep truths about his character as they went through harrowing journeys together. Now he's suddenly forced to work with this new person, when he is only interested in reading about the old one. The reader puts the book down and leaves.
So here's my question: is there some tried-and-tested method for dealing with this? Suppose you write one novel, and then shift to the protagonist's best friend for the next novel. You could theoretically lessen the impact by making the friend a strong secondary protagonist in the first novel, so let's make it worse. What if you're telling a genealogy story, and the next novel picks up with the protagonist's kids? You can't exactly make a one-year-old a secondary protagonist before-hand.
How can you shift to a new protagonist in the next novel?
Note: Ignore killing off the protagonist. This is for if he's still alive and well.
Additional Note: I develop my characters so that the reader cares about them. To me, every protagonist needs a reason for the reader to want him to win. I call this quality Strength. He also needs inner conflict, something unresolved inside of him that makes him endlessly interesting to read about. Inner conflict is usually resolved at the end of the book, but you still have Strength, drawing the reader back to the old protagonist.
The opening pages are very important, because if the reader likes the old protagonist (now a side character) more than the new one, he could easily turn against the new protagonist, which would completely skew your novel.
Do note that this method of character development is my own personal method. It is not part of the question (nor should it be part of the answer), and I only include it to show you where I'm coming from.
I have marked what's reply as the answer, mainly for the excellent outline it provides which I consider very useful. I wanted to note however, that I found part of Lew's answer also incredibly insightful. I wanted to note it here for anyone else who might have this question:
If your story is character-driven, switching protagonist probably makes little or no sense, unless the person is killed and someone else has to carry the torch (but it is not the case, I understand).
If your story is plot-driven, you can pick a new protagonist every time the story requires it. It is your story and you can tell it any way you desire.