A Retool is a toy that has been created by modifying an existing mold to give it new features. This enables companies to create new toys without producing an entirely new mold from scratch.
The Transformers toy lines have often made use of this design choice, in addition to Repaints and Redecos, in order to create new characters or new versions of characters by giving them different features such as brand-new heads.
References
Retool Wikipedia(Text) CC BY-SA