Is there a formula I can use that will change all cells with one character to something else?

For example, I have cells with single letters and no matter what the letter is I want that cell to contain the word Member.

More Info: I get spreadsheets that contain, up to 40,000 rows. Column B will have names in the cells. Every once in a while a column will just have an initial instead of a full name. I'm looking for a way to change every single cell containing only one single character to the word "Member." The cells that need to change could be any letter but no matter what that letter is, if it's just a single letter in a cell, it needs to change to the word "Member."