Tenkousei Sayonara Anata

After his parent's divorce, Kazuo Saito moves with his mother from Onomichi and must leave his girlfriend behind. At his new school, Kazuo is surprised to reunite with his childhood friend Kazumi. He feels embarrassed around Kazumi, who talks about their early childhood stories without any reserve. One day, Kazumi invites Kazuo to her house. After talking about old times, they walk to a place from their childhood called the "Lonely Watering Place." Kazuni tries to scoop up some water with a dipper and the two accidentally fall into the water. They quickly crawl out, but suddenly realize that they have switched bodies!

(Source: MyDramaList)