If all you want is the reversed operation of the existing circuit, simply lift the buzzer, R7 and LED (as connected) so their existing connect to pin 3 goes to pin 8 instead and their existing connections to ground goes to pin 3.
The circuit is a little odd, why they decided to switch the supply to the 555 instead of using the reset pin is strange. It can't be to save power because the IR emitter is always on anyway.
The circuit may still trigger in the presence of other light sources because the IR detector is not particularly selective, it can see more than just the light from the emitter LED. A far better and simpler method is to use the 555 as an oscillator then use it to drive the emitter LED. The detector can then be changed to a standard remote control receiver IC (TSOP series for example) which contains filter circuits to exclude unwanted signals.
Brian.