Extensive UI Changes - Best Practices ?

[Mike Smith]

I am hoping someone here and point me in the right direction, a customer using RS is asking for basically an entire new UI / UX for the application. 

Being on the design side of the house, I am looking for some direction or experiences that anyone else tasked with this might have had ? 

I will have a developer available to build the underlying code but an really looking to understand the interaction between the UI functionality and the code doing the work.

Most of my experience here is with another platform that the web layer called every function by API, and while I think I am right that does not appear to be the case with RS. 

The short question is what would be a best practice to make this application look really different and still work ? 

Thanks