Change from using the external crystal to the internal oscillator
What do I need to change to use the internal oscillator?
  


You'll need to change the fuses on the chip, and you probably want a new 8MHz entry in boards.txt so that timing functions still work as expected.


  
  Can I leave the external [resonator] in place or do I have to remove it?
  If I do leave the external [resonator] in place and set the fuses to use the internal oscillator, will the [resonator] continue to resonate or does it get powered down somehow?
  (I wish to know this for EMC reasons)
  


From the datasheet:


  If the Internal Calibrated RC Oscillator is used as chip clock source, PB7...6 is used as TOSC2...1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.


Also:


  When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port B. A crystal can then be connected between the TOSC1 and TOSC2 pins to serve as an
  independent clock source for Timer/Counter2. The Oscillator is optimized for use with a 32.768kHz crystal.


Therefore if AS2 is not set, the two pins act as normal GPIOs and the resonator is disabled.