The answer to this question depends a lot on where and when you can insert a script. If the script runs before the element is loaded into the page, then you will not be able to find it because it does not yet exist. However, if you are able to load the script inside the body either before or after the element has loaded, you can run a function with the body onload event (if before the element is created) or you can run it as the page is loading if the element has been created. Not withstanding the question of what code you need to garner a DOM reference to the element, the rest is easy. You just change the attribute on the element in question.
$("#foo").attr("type", "checkbox");