Install the injectors right before having the tune put on, or if you can, there is plenty of time during the flash process to install the injectors if your APR dealer is willing to let you. If you don't install the larger injectors your car will just stall out during start up after the flash is put in place... the car can't adjust to bring idle to a level state, ask me how I know

The APR dealer I visited for my Stage 1+ was kind enough to both put a fan on the engine to cool it down so I could change the injectors and also helped me out with getting the first injector off since I was having a problem with it. BTW the retaining clip pushed in... not out. After that it took no time at all and once replaced the car started right up no problem. Good Luck and enjoy the tune.