How to get old Value with onchange() event in text box

You'll need to store the old value manually. You could store it a lot of different ways. You could use a javascript object to store values for each textbox, or you could use a hidden field (I wouldn't recommend it - too html heavy), or you could use an expando property on the textbox itself, like this:

<input type="text" onfocus="this.oldvalue = this.value;" 
onchange="onChangeTest(this);this.oldvalue = this.value;" />

Then your javascript function to handle the change looks like this:

Source : http://stackoverflow.com/questions/1909992/how-to-get-old-value-with-onchange-event-in-text-box