Change in DS-160 Location

 You do not need to fill out a new DS-160 form if your parents have already submitted one with the Islamabad Embassy location and are now attending an appointment at the Karachi Consulate. The US Consulate will be able to access the existing DS-160 using the barcode on the confirmation page, even if the location differs from what was originally indicated on the form[1][2][4].

However, it's essential to ensure that your parents bring the confirmation page of the original DS-160 to their interview. While they can use the same DS-160, it's advisable to inform the consulate of the change in location during the interview process. This helps avoid any potential delays or complications[3][4].

Here is paragraph from State Department's Website.: 

DS-160: Frequently Asked Questions

The Embassy or Consulate at which you actually apply should be able to access your form using the barcode on your DS-160 confirmation page, which you must bring to the visa interview. For example, a business traveler intends to apply for his visa at the U.S. Embassy in City X, so he selects City X as the location where he will submit his application when he completes his DS-160. He then has an urgent reason to travel to City Y on business. Because there is a U.S. Consulate in City Y, he schedules an appointment for a visa interview there, using the barcode from his completed DS-160 application for appointment scheduling. The U.S. Consulate in City Y is able to accept his DS-160 even though it lists the U.S. Embassy in City X as the location where he originally intended to submit his application.

Citations:

[1] https://redbus2us.com/visas/usa/apply/change-ds-160-location/

[2] https://www.boundless.com/immigration-resources/changing-form-ds-160-after-submitting/

[3] https://www.reddit.com/r/USVisas/comments/1bz1gw8/can_i_make_a_new_ds160_if_i_want_to_change_the/

[4] https://www.ustraveldocs.com/vn/vn-gen-faq.asp

[5] https://immigration.dinsmore.com/faq/travel/tips-for-completing-visa-application-form-ds-160

[6] https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/forms/ds-160-online-nonimmigrant-visa-application/ds-160-faqs.html

[7] https://www.ustraveldocs.com/vn/vn-niv-deliveryaddressmodify.asp

[8] https://travel.stackexchange.com/questions/98510/ds-160-form-b-2-visa-travel-booking-procedure-and-answering-travel-informat