Insert a div element as parent

I'm just wondering if the following is possible, lets say we have a dom element and we want to wrap this element in a div. So a div is inserted between the element and it's parent. Then the div becomes the element's new parent.

But to complicate things, elsewhere we have already done things like:

var testElement = document.getElementByID('testID')

where testID is a child of the element to be warapped in a div. So after we have done our insertion will testElement still be valid?

BTW: I'm not using jquery.

Any ideas?

Thanks,

AJ