There's two ways you can do this cleanly. One is to start from scratch and create a GraphQL server that directly hits your data source(s) by replacing the REST component of it. The other is to create a GraphQL schema that serves as a gateway to your REST app. In both solutions, you'll be creating GraphQL types for your input and return types, and re-writing your calls on your client apps to conform to GraphQL. However, the long-term benefit of this is that you can write your queries once, and run them anywhere on any client app platform.