what if I arrive a day earlier than what's on my visa due to a flight change?

If the day is included in the visa validity, then you can enter that day.

Usually, you can enter China the moment you get the visa and the visa is valid till the "Enter Before" date on the visa. Check your visa again to see if you have misunderstood the "Enter Before" date or not. Remember you must enter before the "Enter Before" Date.