You can even replace most CMOS batteries on a running machine without causing any issues. The power from the system will keep the BIOS set, and it's a 1 minute job for many servers, slide out the rack, pop the top, thin phillips screwdriver to the CMOS battery to pop the old one out, pop the new one in and button her up.

Heck, you can buy 2 CMOS batteries for a buck at the dollar tree. They'll last a year or more with the light usage that would be.