I've heard advice that you should change at least three aspects of a person if you base a character on them. I'm not a writing a memoir, but I plan to use heavily my own experiences, and changing my ex's gender, religious and political views, or diagnoses would have significant ramifications for the story I want to tell.
Prose based on fictionalized yet real experiences really does ring truer for readers. It's why I feel that it's extremely difficult for (most) very young writers to produce realistic, relatable characters and stories. They simply haven't lived enough life to make worthwhile commentary on it. I don't think you should limit yourself a ton when creating characters based on real people, provided that they are no longer going to be a part of your life. Family members? Hell no. Exes? Of course, as long as you aren't in communication with them and won't be considering getting back together in the future. Just don't use their real name, and don't write anything they've texted/emailed/etc. to you verbatim as that could open the door to legal nonsense if they happen to read the story someday.
If there's no identifying information, ie. name, address, date of birth, etc., I don't see how it'd be a problem.
