Is a crystal needed for burning firmware on AVR?
You must have an oscillator as prescribed by the current fuse settings in order for programming to work. If the current fuse settings specify the internal oscillator then you can program the fuses to use an external crystal instead, but once you do so you will then require an external oscillator in order to perform subsequent serial programming. Parallel programming will still be available, since it is clocked independent of the system oscillator.