The standard instructions for changing out a desktop CMOS battery say you should turn everything off and unplug the power cord. But I wonder if guys who do this kind of stuff for a living don't hot swap the batteries with the power on, or at least leave the power cord plugged in so the always-on 5V supply provide power to the CMOS. That should prevent losing the contents of the CMOS memory, including the TOD and the BIOS settings.
On a related question, I also have an old Toshiba Satellite L35 laptop that I'd like to replace the CMOS battery on. It looks like I would have to take the entire laptop apart so I can remove the motherboard, turn it over, and replace the battery. Or, it turns out I can see the battery through the side of the wireless compartment, and I should be able the cut a window in the bottom of the case through which I could switch out the battery, then hot- glue the window back in place. Is there any reason not to do it that way? It's a lot more likely the laptop would survive the latter procedure.