I've always turned the power off just on the car I am working on.
When I change the power board I pull all the other cards out of their sockets. I install the power board, power the unit up and check my voltage from SP to SC on the power board to confirm 5.05 VDC. If the voltage is lower or higher adjust the pot on the power board to correct. After that I pull the disconnect, reinstall all the other boards and power the unit back up. Check your voltage 1 more time between SP and SC to confirm voltage hasn't dropped after installing all the other cards.