Displaying a hidden field, once it has data.

I have a radio button used to show/hide a comment field.  Initially the comment field’s presence is “hidden, exclude from layout” so that when the form opens, the
comment field does not show.  When the radio button is checked, the comment field then displays. Problem:  Once the user selects the radio button and enters
data into the comment field and saves it, how do you get the comment field to display every time the form is opened thus ignoring the “hidden, exclude from layout”
setting?  

Thank you