IR Emitter/Detector with 555 timer alarm
R5 will definitely need to increase to keep the LED (D2) from having too much current. You can probably make this a 680ohm. The QSE113 photodiode will still largely produce the same current into the base of Q1 for the same light so no change envisaged at this point. I think I'd be tempted to add a resistor from Q2 base up to the positive supply rail - make it a 10k but be prepared to increase it should sensitivity be a little too low.

I don't particularly favour where R6 is at the moment - instead of it going to the positive rail, try conecting that leg to Vcc on the 555 and checking things still work OK at 5V, then it should be OK at 12V. Where it was connected and with Q2 switched off (Dark) the 555 would be unpowered except for the R6 connection and this might do some damage in the long run.

Make sure you have a 555 that is suitable for 12V and you should be OK.