Can a PO req be re-approved if the amount i changed to a higher amount

I have a request to setup a workflow where an approved PO Req amount has increased.

I can see where i can set a parameter to compare the line extended cost to the Originating line extended cost.  but if the PO is already approved will this trigger a new approval needed?  Also how can i get a dollar amount threshold.  example: if the difference is greater than $100.00