There is nothing "backward incompatible" specifically in this use case. Understand the meaning of "contract". If your system is exchanging information with others, you can't just make changes in the contract without letting others to acknowledge and accept in the name of "backward compatibility". Just because it's an effort investment to make changes at multiple places doesn't mean that it's wrong.

Still if you want to make your contract scalable then you need to send additional mapping in each of the response.