Puppy food change - diarrhea

Puppy is 4 months old and has been on Royal cannin puppy golden retriever food. I accidentally ordered a bag of Labrador retriever puppy food when we ran out of the previous bag. So we switched him abruptly to the new bag. It’s been 7 days since and he started having diarrhea today - is this from the sudden change in food?

Should I get the golden puppy food bag today and switch him to that? Or get a bag and slowly transition him to the golden puppy food ?