Design Issue: Should UI layer worry about it's backend?

Design Issue: Should UI layer worry about it's backend?

It is more of a design and architecture question.

I am developing a new UI layer for an old system. This system accepts
request in particular xml format. Currently request from the new UI layer
goes to a data massaging class via controller.

This massaging class converts UI request xml to desired request format. It
adds few deprecated elements and constants to the XML it received from UI
layer.

Request XML from UI is partially similar to the actual back end. But it
has to go to the massaging class to be converted into actual request.

My question does UI layer need to worry about if it's request XML is
partially similar to the actual request? Can UI layer just send the data
in JSON format to the massaging class and massaging class will convert it
into the actual request xml?