First, a journal is not a rough draft of a memoir. It is what writing theorists call "prewriting". You write regularly (ideally every day) on whatever comes to mind without self-editing or censoring ideas. The purpose is to capture your thoughts and gather ideas. A journal is a notebook for you to jot down things that flash across your mind. Some entries may recount events others note topics for later reflection. A journal is often fragmentary rather than being a finished piece of writing.
A memoir, on the other hand, is developed by looking back upon what has happened and how you felt about it. It is a process of reflecting on events, the emotions they triggered and the lessons they taught. A well written memoir is carefully planned. You select those slices of life which seem most important and look for the insights they provide. A good memoir may draw on many of the techniques of fiction to tell its story.
It is important that you understand that a journal is only one of the types of prewriting that you might use to help create a memoir. There are a variety of other writing exercises that help generate the stories and ideas you will ultimately incorporate into the finished memoir.