Migawari Koutei no Yuuutsu: Kokyuu no Jijo desu ga, Irekawatta Koutei ni Subete wo Oshitsukerareteimasu

Shirai Kasumi, the daughter of a rural nobleman, is a maid in the harem serving Seiran, a consort candidate for the Chaoyang Kingdom. On a full moon night in mid-spring, while trying to save Emperor Tenyou from falling into the harem pond, they both fall into the pond, and their bodies are swapped! Until they can carry out the proposed solution of "trying to fall into the pond together again on the next full moon night," they must actually live with their bodies swapped... After accidentally becoming emperor (on the outside), Shirai Kasumi finds out that the consort candidates are underestimating Tenyou because the empress, Hidemasa, is too strong. In an attempt to rectify this, Shirai Kasumi devises a "Plan to Make the Empress Grateful," but...!? (Source: Ichijinsha, translated)