Put an alert on the item? That still requires editing the item on the existing order. If 'Auto edit items' is defined in File/Options, it is easy to cycle through all items on an order, one at a time.

There is no 'automatic' way to warn if the pricing on an order is different than what is set on the items. In many cases, one wouldn't want that because they have over-ridden the price and a bunch of messages that the prices don't match would be very annoying.