I do not see an option to modify this field as it is a result of other fields and has always been a display only field.
The calculation of this field is based on the delivery date of schedule line and planned delivery time with respect to the plant calendar. more details in Q&A 2 of OSS note 498143 - FAQ: Stock transfer with delivery in purchasing
MAybe you can explain some background to the request, there might be other options. Do you get anywhere an error, e.g. in delivery creation if you create the delivery at a different day than the expected?