I have a server that has a single power supply that it is plugged into the wall. I want to transfer the server over to a UPS power supply for redundancy but do now want to power off the server for the move. The UPS is plugged into the same power source (phase) as the computer is powered on. Is there a way to do this?
Just schedule downtime and turn the server off.
Without a dual power supply setup, you don't have any other reasonable options.
If there are business constraints or otherwise, you should note those in your question. Any sane organization would understand the need and justification for the power off. But I'd assume that if the server were absolutely critical, it would have dual power supplies or wouldn't be plugged into the wall in the first place.