How do (or can I even) change part of my liquid starter to a sponge starter?
What exactly are you trying to achieve?
A sponge is basically a pre-ferment. The Italians call it a biga. the French call it a poolish. Turning a starter into a sponge would mean adding some yeast when you feed, so you'd probably want to divide the starter in half, keeping the "clean" starter separate. You'll get complex flavor with less sourness, and it will be treated like a yeasted dough for the most part.
it has some recipes that you use a liquid starter and some you use a sponge starter.
All I have is a liquid starter, but there are some recipes that he has that only use a sponger starter.
Having not read Ed Wood's book, I really couldn't say what he means. From what I gather, sponge means pre-ferment, which is just a method of preparing some of the dough early and letting it sit to form more flavor. I could well be wrong, it's one of those terms with a meaning that is never explained perfectly and which seems to encompass several different ideas.
Don't know if you've moved past this or already found the thread, but if not, here's a link to a previous discussion on these terms: