You would have to store the previous value somewhere. Maybe when the page first loads you could add a custom property:
<input type="text" value="10" originalValue="10" />
Then in JavaScript read it:
var sOldValue = textbox.originalValue;
There is no built-in way to retrieve changed values.
Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/