[balloonknott2006]"I've created my own text/shape in Photoshop, but when I add it to the Library, Or the Shape folder, it won't let me replace it.
(I hope I'm making sense)"
I'm not sure what you mean, but it sounds like you're almost there. There is no "replace" button for library shapes. Just add the new shape to the library, then select it, then click the "make active" button.
As a reminder, any changes you make to the library emitters are permanent. In this case you probably do want to make a permanent change (since the word "illusion" or "pIllusion" won't be useful to you), but whenever I want to make changes to an emitter I add it to the stage, make changes to the stage emitter, then use "Add to Library" when I have it the way I want it. This way I always have the original library emitters.