You don't have to change your username to have your display name be your real name.
Near the top of the page, click PROFILE. On the following page, on the left side under "Modify Profile", click "Account Related Settings." Then you can change the Display Name that people see.
For example, I still log in as elecuyer (my std. username for all discussion groups) but my display name (on this forum) is 'Ed Lecuyer'.
BTW, I agree 100% with James that using/displaying your real name is very beneficial. It helps people who don't get to Sheepscot very often "know" each other when they do come. (No one would recognize "narrowgaugetraingeek123".) In addition, it eliminates inflammatory posts. While this was very rare on the old WW&F forum, it is very common in the railfan community and many discussion forums in general.
Of course, sometimes I think I'm better known as "Emily's Dad" - but that's besides the point