Where did you come across such scenario?
Are you serious with that question: "do we need to change the PO value"?
You price in the PO is the price that you agreed with your vendor. How can you assume that this price has to be changed? This price must only be changed after having contact with your vendor and having negotiated and agreed to a new price.
What you see in material master is your valuation price, in most companies it is a moving average price in case of purchased materials. But some might have a reasons to use standard prices e.g. to avoid heavy changes in their finished goods calculations when there is a bigger fluctuation of market prices.
And if this standard price is set with a standard cost estimate, then there is a valid from and valid to date to this estimate. And the variance between PO and standard price is posted to a difference account.