In CRM 2016 with SP1, it seems that a "bug" has been introduced where the price of an order product added from the catalog (an "existing product") cannot be changed. This was not the case in earlier versions.
A field on the Order Product form, named Pricing (internal name ispriceoverridden) determined whether the price per unit can be changed. However, on the Order Product form this field is set to read-only with a value of "Use Default"; so there is no way to change the value to "Override Price".
Starting with CRM Online 2015 Update 1 and CRM 2016 On-Premises Export to Excel behaves differently.
The dialog box which offered the setting "Make this data available for re-importing by including required column headings" no longer shows.