Yes. You can make changes after you certify the list but if you make changes you still then again have to certify it again.
Here's a quote from the official NRMP page
Certifying the rank order list is the final step in the ranking process. By certifying their lists, applicants indicate that the list is complete and represents the final data that should be used in the matching process. Only certified lists are used in the Match. When applicants have finished entering their lists, they must certify them by clicking the Certify List button located on the My Rank Order List screen and then entering their NRMP password to confirm it.
Changes still can be made to the rank order list after it has been certified. However, once the list has been changed, the new version must be certified in order to be used in the Match. Changes include adding a program to the list, deleting a program from the list, and changing the order of the programs.
Please note: The R3 System does not retain previous versions of a rank order list regardless of whether the list was certified. The list currently displayed on the screen upon logging in is the only version the NRMP has on file. After the rank order list deadline, the NRMP will not add, delete, or move programs or in any way modify a rank order list.