I wouldn't toss the food you have as it's decent chicken food, but when it gets down to the bottom of the bag, I'd start mixing it over a few days. I doubt they'd have an issue switching cold turkey, but it doesn't hurt to just ease them into it to be on the safe side.
We feed ours Kalmbach's crumbles which is a much more expensive feed and it's 16% protein. I buy their Flockmaker (20%) too and mix it, plus put out the oyster shell.
We buy the crumbles as these are silkies and don't eat the pellets well. Your brand probably comes in both as does ours.