Thanks, Jenny. As a general rule, under U.S. law, you can't libel the dead.

The challenge is that if you disparage them, the heirs may come after you for other legal reasons: Let's say, you quoted from letters that someone else had the copyright to. So you have to be careful.

The issue of how much you have to change is too complex for a comment, as is that of memoirs (which raise their own issues). As a nonlawyer, I'd hesitate to give opinions there. It's good you're thinking about the issues, though.