Keeping the main battery powered while changing the 12 volt? Need help

So my 12 volt battery is on its last limb. Got a brand new 12 volt from Toyota OEM. However, in my owner's manual it doesn't say anything about keeping power on the main battery while you change the auxiliary battery. However I've heard from people through word-of-mouth that you have to keep the hybrid battery powered with jumper cables to another car or you will lose radio presets. I haven't seen this in the owner's manual...