Updating Volunteer's Addresses

We're hoping to mail out some appreciation gifts to our volunteers this Fall, but we're realizing that many of our volunteers have not updated their addresses in some time. Many of our volunteers have very low technical ability with computers, so asking them all to update their address through their accounts will leave us with hundreds of out of date addresses that need to be updated manually on our end. We're looking for ways to cut down the amount of work this would take, and one idea was using a spreadsheet to compile the list of volunteers and their updated address. Is it possible to upload data to overwrite existing data in CERVIS, or would this risk corrupting/losing existing data? Does anyone have experience with this kind of process?