It's better to replace as you go, since finding an issue after many tiles are installed after the affected tile may require dismounting many of the affect tile's surrounding tiles (it's harder to get an LED Module in place if it has other tiles in place around; less flexibility).
The process can be done with the power off, however, the installer will not be able to distinguish which LED Modules are damaged, or have damaged LEDs. This will lead to potentially requiring replacement for not just the affected LED Module, but also the surrounding LED Modules.
Note: LED Module on the edge of the display allow for the most flexibility. LED Modules having as little modules surrounding the LED Module allows for an easier fit.