Not sure if I understood you completely. But tableau won't ask for a new extract while importing the old workbook. You might wanna work as below:
(1) Create a twbx file with old extract
(2) Create a twbx file with new extract (but make sure all fields are same).
Import the (1) workbook into (2). You would see 2 datasources into the new file which is (2). As a final step, we would need to replace the old extract with new one.
You're done! Let me know if that helps!