XUL Overlay question

[Philip Reames]

How do I replace an existing element? (IE. remove it and add one with
the same name)

Here's my situation: I'm trying to modify the collapsedHeaderView in
msgHeaderOverlay.xul. This id is hard coded into several javascript
files including msgHeaderOverlay.js, so I have to modify it rather than
hide it and insert another element. I would like to be able to create
an overlay of msgHeaderOverlay.xul which removes the current
collaspedHeaderView element and adds a new one of my own construction.
Is this possible? And if so, how?

Yours,
Philip Reames