I accidentally changed my dogs food!?

I usually buy him the Science Diet adult light food but i accidentally bought him the Science Diet adult food (not the light one) and he's been on it for about 3 weeks now and I've noticed he's been farting alot. lol. Should i stop feeding him that and buy the right food or should he finish up that food and then just go back to feeding him the right one or should i give him the right food gradually like mixed into the food he has now? And if it helps, he's a medium sized dog. :) Thanks.