The only way to get the radio code is to get the serial number. It doesn't require the complete removal of the radio but you need to remove the center console among other things to see what the number is.

I have seen some products out there that plug into your cigarette lighter that keep a low current going through the car so that the radio codes, ECU memory, etc aren't wiped out when the battery is removed. I would search around for one of those and it should give you enough time to swap the batteries out and still spare you the hassle of obtaining the serial number.