"The Ethicist" column in today's New York Times magazine addresses the issue of whether to tell your friends that you are removing them as guardians of your children under your Will. The column is here.
I'm not an ethicist (insert attorney joke here), but I agree with Randy Cohen that there is no need to tell the friends about the switch. Especially when you are switching to family members (most people understand that blood is thicker than water). Maybe I feel this way because my wife and I did something similar. We named our friends as guardians of our daughter, but then switched to my sister and her husband once they got settled and had kids, and we saw that their parenting style matches ours.
A similar ethical issue (not discussed in "The Ethicist") involves telling people that they are named as guardians. I'm always surprised that people DON'T tell their friends/relatives that they have named them (or plan to name them) as guardians. I know this always makes for a heart-warming film ("lovable moppet(s) show up at the door of self-absorbed yuppie, who then discovers the value of family"), but it's significantly less heart-warming in real life. My advice: talk to those you plan to name, BEFORE you do so. (They may say no. That's what happened to the people my in-laws asked, when my wife was a kid.) And talk to those you have named, AFTER you do so, to fill them in on how things will work.