If I'm programming a pin change interrupt I can call attachInterrupt at various points in my code and change what function is called when the interrupt occurs.
The overflow and compareA/B interrupt calls are defined by the ISR macro which is evaluated at compile time, which therefore can't be changed at run time. One solution would be to make that function consist of a single line, which invokes a function pointer. If I'm reading the code correctly, this is how attachInterrupt is implemented. Short of mucking with the interrupt vector table (which seems like a bad idea, or at least very hard to debug if you screw it up), is there another way to redefine the overflow or compare interrupt functions?