No. What you need is one spot warm enough on your coldest days and spots cool enough on your warmest days. There is no reason that the heat has to drop. The chicks can handle cooler temperatures as they feather out but that does not mean they cannot continue to handle warmer temperatures.
Think about a broody hen raising chicks in the real world during summer. The days do not have to get cooler as the chicks get older. If you want to lower the heat you can but don't feel that you have to. Keep it as simple as you can.
Excellent, that is cool enough. Sometimes I have frost on one wall of my brooder in winter but I also have an area that is toasty warm. As long as they have a warm spot to go to they will be fine. How cool does that get when your temperatures go up? I've had outside temperatures go from below freezing to the mid 70's Fahrenheit in a day. That's why I say they need a cool enough spot.