Can I hot-swap a 2-PSU server from 120 to 240 volts?

I'm migrating a server with 2 PSUs from 120 volts to 240 volts, and I'd rather not take it offline. It seems like I should be able to just move the PSUs to the new voltage one at a time, since they are not connected to each other.

Would it cause problems to run one PSU on 120 volts and one PSU on 240 volts for the time it takes to rack a PDU?

You can, but you probably shouldn't.

Can you try it with a less important server? Also, the make/model of the system may make a difference.

Edit:

I'm shifting a rack from 110V to 208V and decided to give this a try. It seems to work on my HP ProLiant servers without issue.

Assuming the power supplies support both voltages, then it will not be a problem. The PSUs output the same DC voltage either way.

No. It will not cause problems to power one PSU at a different voltage for any length of time, including indefinitely.

It will make load calculations confusing if you plan to run like this indefinitely, but the servers won't care at all.