Are function pointer assignments atomic in Arduino?

The following snippets are from TimerOne library source code:

// TimerOne.h:
void (*isrCallback)();

// TimerOne.cpp:
ISR(TIMER1_OVF_vect) // interrupt service routine that wraps a user defined function supplied by attachInterrupt
{
  Timer1.isrCallback();
}

// TimerOne.cpp:
void TimerOne::attachInterrupt(void (*isr)(), long microseconds)
{
  if(microseconds > 0) setPeriod(microseconds);
  isrCallback = isr; // register the user's callback with the real ISR
  TIMSK1 = _BV(TOIE1); // sets the timer overflow interrupt enable bit
  resume();                                                                                            
}

The question: if the timer is already running, and the main program calls attachInterrupt(), could the timer interrupt occur there during the function pointer assignment isrCallback = isr;? Then, with lucky timing, Timer1.isrCallback(); function pointer would consist partly of the old and partly of the new address, causing the ISR jump to a bogus location?

I suppose this might be the case, since function pointers are certainly wider than 1 byte, and accessing > 1 byte data isn't atomic. Possible workarounds could be:

• Always call detachInterrupt() to make sure the timer isn't running, before calling attachInterrupt(), i.e. clarify Timer1 docs.
• Or, modify Timer1, disabling timer overflow interrupts temporarily just before isrCallback = isr;

Does this make sense, or is there something in Timer1 sources or function pointer assignments that I've missed?