There are a few commands for changing various properties of an account (i.e., the relevant field in /etc/passwd):
Change the full name field.
Change the login shell.
Change the password.
Other tasks need to be done by hand. For example, to change the username, you need to edit /etc/passwd directly (with vipw, remember). Likewise, to add or remove the user to more groups, you need to edit /etc/group (with vigr). Such tasks tend to be rare, however, and should be done with caution: for example, if you change the username, e-mail will no longer reach the user, unless you also create a mail alias. 
The user's name might change due to marriage, for example, and he might want to have his username reflect his new name.