This should be extremely easy to perform.

What I want to do is:

When someone visits a page on my website and for example it loads the page normally and the body is like.

The user GlitchKing is the webmaster of this site and GlitchKing is cool.

When loaded with the new replace script or w/e I need the body would say:

The user GK is the webmaster of this site and GK is cool.

So basically everytime the word GlitchKing is used on my site it automatically replaces it with the text GK.

Thanks in advance!

1. use a automatic replacing tool to replace the text in all your files.
2. use javascript to replace the text ( str.replace( look-for-this-text, replace-it-with-this-text ); )