By the time being the RadSpreadsheet exposes only the Worksheet.Cells.CellPropertyChanged event which can be used to track if a some cell property is changed. But you can not get the old value from the event args.
A possible workaround is to track all changes and preserve them in some data structure which will allow you to get the old values.
We will consider implementing this feature for our future releases. I created an item in our Ideas & Feedback Portal where you can track the status of the feature Implement option to track changes of cell properties
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