Purchase order re-approval on changing

Hi,

I have a purchase order approval path set up and want to know once a purchase order is in approval - if a purchase order value is then changed, how can I get it to go through the purchase order approval path again.

for example, if a purchase order is raised and approval but then the purchase order value/quantity is changed - how can the system be configured so that it required approval again from the existing "approval path" setup.

thank you,