I have 2 hosts with Nvidia GPUs under driver version 260.99. Due to power supply considerations, and to facilitate a future upgrade, I would like to swap the cards between the hosts. Under these circumstances, will it be necessary to uninstall and reinstall the same drivers?

You should be ok swapping the GPU's without touching the drivers.

Yes as Bruce says, the only thing you may have to do is Re-Boot several Times. The first time you Boot up with the Different Cards the OS will Discover them as New Cards even though their the same Type Cards. The system may even tell you to Reboot again, if not I would do it anyway just to be sure ...