What's the nature of the changes that cause you to rewrite large swaths of your frontend js code? Is it new data that you need to surface in the frontend, or is it changes to the structure of the data?
If it's new data and you want to stop focusing on updating your frontend code to deal with it, you would probably need to implement something that lets the javascript build out your frontend in a more dynamic way. I could see this working as a service that passed back as a structured UI mapping in some data format that your frontend js could parse (you'd have to include all the data you needed and probably some information about the format of that data, ie, string, text, date, etc).
That might work alright for some form data, that's basically how form objects work in MVC frameworks like CakePHP or even Drupal. And in fact if that's your goal, to just provide some user-facing content entry, you might even be well-off to check out implementing this code in one of those frameworks.
If the problem is that you're making changes to your structural data, but by and large you're surfacing the same data fields, you probably just need an abstraction of your front-end data models and your backend data models. You can come up with your javascript object definitions that define what the structured data you pass back should look like, define what your backend model looks like, and then define a mapping layer between the two. If the structure of the data changes in the backend, your contract between the javascript object definition and the mapping layer wouldn't need to change, and you could merely change the contract between the mapping layer and your backend data model layer.
