I have been reading a lot of memoirs recently, and none of them do what you are suggesting. I can't say it never happens, but I don't think it is as common as you are implying. I will say it again, the more you change the less it is a memoir.
How do you know none of them do this? Unless the author says so or unless someone from the book comes out and says it, you won't ever know. But I'll give you examples from four best-selling memoirs that indeed do what you say isn't common (or worse). These all go beyond what I'd feel comfortable doing myself.
Frank McCourt (
Angela's Ashes)
He totally made up a person:
"Their three biggest criticisms of the book, aside from the endless grinding misery it depicts, include the description of a local boy, Willy Harold, as a Peeping Tom who spied on his naked sister. It turns out that Mr Harold, now dead, never had a sister -
which McCourt did later acknowledge."
Another article, which I can't find at the moment, said that one of his characters was actually a combination of various people, not truly one person.
Mary Karr (
The Liars' Club)
She attributed an act of one person to another person and says that she used fake names (I'm okay with the latter).
"This is the only intentional falsehood I've consciously constructed—other than fake names. It's the one time I've let literature rule over fact. And now that Meredith and her mother are both dead, I correct the score.
Oh, and the Liars' Club stories in
that book (minus one I'd tape recorded) were sheer fiction, but since they deal with frozen farts and the like, I figured their historical accuracy would never be under dispute."
"The greater complaint has been that I
didn't use real names or the real name of our town. In other words, people preferred to be affiliated with their representations in the book. Some folks were pissed I left them out."
Jennifer Lauck (
Blackbird and
Still Waters)
She uses some pseudonyms, and she even got her father's middle name wrong, though that wasn't intentional. But, again, that doesn't bother me. What does bother me is her "oh, well" attitude when accused of being untruthful about some events, even after figuring out that some things were indeed wrong.
Augusten Burroughs (he didn't write under his "real" name):
Running with Scissors
He's had a lot of issues, including a family accusing him of libel and invasion of privacy,
but back on topic:
"Additionally, there is an author's note at the beginning of the book saying that 'the names and other identifying characteristics of the persons included in this memoir have been changed.'"
There are many, many more examples. In fact, one of the few that comes to mind who says she
did not change any names is Jeannette Walls (
The Glass Castle). Actually, I think she says she "
could not change the names." I think she initially had considered doing so but couldn't bring herself to do it.
Basically, yes, it's very common to change names. Many people now do it to protect others. As a reader, I'm okay with that. I don't feel cheated. Perhaps I would feel different if I were closer to the story and knew the people in real life, but that has only been the case with one memoir I've read. In that book, the author did use her family's real names, but I have no idea if the names of all others in the book were accurate; I suspect not, though. These days, however, with creative nonfiction, people are taking far more liberties and are creating scenes and dialogue and even characters. That does bother me.
Beyond that, I think the biggest thing to remember, which I've seen over and over again, is that memoir is not history.