There are certainly tales where humans are turned into rats. The Aarne-Thompson motif index, which categorizes fairytale tropes, includes one for “Transformation: man to rodent” (with the identifying number being D117). Sublistings include rats and mice. If you read through motif indexes from different countries, you may find some collectors who list examples.
I located a few stories, most of them obscure. Some were actually about mice, but I thought for these purposes they could still be relevant.
There’s a widespread tale type called “The Animal Bride,” kind of a gender-flipped Frog Prince tale. The enchanted princess can be any kind of animal. In a Swedish version (“The Rats in the Juniper Bush”), she’s a rat. In a Finnish version (“The Forest Bride”), she’s a mouse. These versions don’t really focus on the background. They just mention briefly that the princess fell under some sort of enchantment in the past.
In an Irish story, "Shaking-Head," a servant girl is briefly turned into a rat so that she can steal something.
In a Tibetan tale, “The Story of the Potter and the Princess,” a grateful spirit gives a man a cloak made of rat-skins which allows him to transform into a rat.
- Macdonald, David. “Tibetan Tales, III. (Continued).” Folklore, vol. 42, no. 4, 1931, pp. 447–464. JSTOR, www.jstor.org/stable/1256302. Accessed 4 Aug. 2020.
In Hawaiian myths there are rat-people and a character named Kane-pohihi who takes rat form.
- Beckwith, Martha Warren. Hawaiian Mythology (1940) p. 433-435
In the Ganesha Purana, the celestial musician Kraunca or Krauncha gets turned into a mouse.
And in the fairytale Puss in Boots, Puss tricks the shapeshifting ogre into taking the form of a mouse. At that point, the ogre is easily defeated. This is the most famous tale I can think of where someone turns into a rodent.