When you mix RAM its a crapshoot whether it will work or not. Corsair hasn't been playing nice with the stock Samsung sticks for some reason. The way its supposed to work is if you match the voltage, latency and speed there should be no problems with new sticks.
Personally I go the route of total replacement and just put back the stock ram in the event I need to send the laptop in I can put the stock back. Just be aware that technically speaking, opening any compartment of your laptop with the exception of the user serviceable drive and memory expansion slots voids your warranty. Realistically speaking if you do it carefully and are not using a crowbar to open it up there are no seals.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein