If it is only your login name, then there are no repercussions changing that in System Preferences > Users & Groups > account > Full Name. If you would change your home folder's name, thus the short name of your account, you probably get in trouble.
It is actually easier than you think, as in the above link, open Users & Groups, click the Padlock, enter password, right click on User, click advanced, change both the User name and path to home folder, last step is to go to your home folder and change the name to the one you input in the path in advanced.
Thanks. I double checked. Under the user folder, there is my home folder "Snoopy". I don't need to change that. On the top right of the screen, there is a full name. I want to change it. So, I just change it in: System Preferences > Users & Groups > account > Full Name without messing up the system files?