How is a memoir any different from a film labeled "based on a true story"? And yet, few object when untruths are told there.
The answer is right there, in your words.
A movie "based upon a true story" is not a memoir. It's entertainment, like a novel based upon real events.
The movie equivalent of a memoir would be a documentary.
I have written about this issue before.
At the top level we have major claims about your life. There should be no question about whether you were in prison, in the navy, or if you climbed Mt Everest.
Below that, we have many story elements that will not be verifiable or provable. Did you catch five fish or fifty in that derby back in 1979? There are some who say, "don't put it in the story unless you are 100% sure it's right."
But I have learned that your truth and mine will often differ when it comes to long ago details. You say your brother Ted caught more fish than me. I say Ted wasn't even there that day. Both of us believe our versions to be the truth.
And it gets even more complex with behaviour. I think you got drunk and attacked me. You say we were just drinking buddies, having a good time. But from that day, the course of our lives diverged based upon differing perceived truths stemming from that event.
When you write a memoir, there will be some elements you can verify. For example, when I wrote about being on tour with KISS I did not remember the dates of the shows on the Dynasty tour. But I was easily able to get that info online. If you describe verifiable things like that in your story, you have a duty to get your facts straight.
What about those fish stories, though?
What I did was go back to the people who were there. What happened, I asked? Some said, you had five fish. Some said, you weren't there. One guy said twenty. In that instance, I had to make my best guess.
There were other times when the recollection of my friends showed me that my own memory was faulty. In those cases, I changed my story. There are others who would advise against that, though. They'd say, "Your memoir is YOUR story. If they want their story told, let them write their own book!"
There is some vaildity to that argument but in the interest of being as accurate as possible I chose to go with what I believed to be correct, as opposed to my vague recollection (recognizing that our own recollections tend to always favor 'our side')