Those are your instructions! 
See this thread: Tutorial: Upgrading Factory Reset Firmware
Why would that erase other stuff in memory? I’m confused… following the instructions you’ve posted here don’t work. Following the process I’ve posted here does work (flash new firmware, redo particle setup). I’d like to put down a proper set of instructions in case anyone else is in my shoes with an older core. Spark Core is a great product, but not if people can’t figure out how to use it!
So, I’d politely request that if there’s a better way to set up an old core, those instructions be posted here. If my instructions aren’t acceptable (not sure why they aren’t, I’ve pulled all the info from Particle/Spark sources & this board), then list steps that do work.