I'm going to install a new hot water heater in a business office. I can't shut the water off to the whole office while people are @ work. If I just shut off the valve on the cold side of the tank will I be able to swap out the tank without water everywhere

If you have a shut off valve installed on the cold side yes you will be able to swap out the tank,Here is what you need to do.

Close the cold shut off valve, shut off the gas to the tank, drain the water heater make sure you open a hot water faucet in the house, disconnect the cold and hot water lines, disconnect the gas line, remove the tank, reverse the steps for installation, good luck.

John