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.