Disabling Username in ChangePassword control

I'm using the asp:ChangePassword control.  I would like to disable the ability for the user to enter a new username but the default shows the current user's UserName and allows editing.  Ideally, I would be able to find the child control in the ChangePassword control that is the UserName textbox and set Enable=false.  I cannot figure out how to do that so any help is appreciated. 
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