Not having access to all of the source I think it should be nothing more than changing the template or script that builds this line from:
<input type="submit" name="preview" value="Preview" onclick="return submitThisOnce(this);" accesskey="p" tabindex="3" class="button_submit" />
To
<input type="submit" name="preview" value="Go Advanced" onclick="return submitThisOnce(this);" accesskey="p" tabindex="3" class="button_submit" />
