RAM can be finicky. Mixing kits is not recommended unless you already have both kits to try it because it isn't guaranteed to work. It is best to replace your RAM with a new, larger kit than to extend it with another similar kit. Once you have the new RAM installed, you can try selling your old RAM. RAM tends to hold its value better than most other components anyway.
You do not need to change any BIOS settings or anything like that. Its as easy as removing the old RAM and popping in the new RAM.