RadRichTextBox does not provide an option to retrieve the old value, but you can easily implement this logic on your side, by keeping a field or property, which you update every time the ContentChanged event is fired. The property can be of type string (the exported document as XAML or HTML for example) or RadDocument (by cloning the document when it's changed).
If you meet any difficulties implementing this scenario, do not hesitate to contact us again.
the Telerik team
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