The interrupt vector is stored in the PROGRAM (FLASH) space. This means it cannot be changed during program execution. I don't know of a way to implement an ISR for timer0 using the Arduino IDE. Also, I don't know how you came up with 976,56 Hz (976.56 Hz for the rest of us), but the prescaler is set to 64 so the frequency is 16.000.000 / 64 which equals 250kHz.