Yes, you can modify a personal letter after it has been already assigned to a program but this can only be done until the closing of the application period. Once file review has opened, you can no longer modify or un-assign a personal letter.
Please note this is not the case for any Post-Match Process (PMP), as there is no file review period. Once an application is submitted during the PMP, it becomes available to the program. It then follows the same rules as though a file review period has begun.