Increase PO value without constant change orders

Summary:

I am seeking possible solutions for a frequent issue associated to purchase order change orders. End users create a Services requisition and purchase order for a specific dollar value that matches Supplier's services quote. Then work changes and the PO value needs to increase. This often occurs multiple times on each purchase order. It is not an option to increase the initial value of the purchase order as we cannot send a supplier a PO for more money than what is quoted. However, we require numerous change orders as the value of the work increases.

What options are there for adding additional funds to a purchase order without requiring a change order for every increase? It requires multiple levels of approvals and is time consuming. This is a common problem that large companies have and hoping that other companies have found an adequate solution.