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.

chris is right, never replace a power card in a duplex system without shutting down both cars, It is in the manual, and i have personally experienced what happens if you don't follow this advice. turn both cars off, remove power card you want to replace, install new card, and turn on, adjust 5 volts as Q says, i use a little wooden stick to adjust the pot, adjust for 5 volts. the other pot is the motor limit timer pot, I usually just turn it to its max setting, you will know which way is correct, cause it will shut down on low oil timer if you turn it the wrong way. thats about it.

Thanks for your responses

Thanks for the info Scott. I was unaware of that procedure on duplex's.