Firstly let's talk about the differenet models- Requirement model, Analysis and Design model, Implemenataition model, Deployment.
When we talk about the
Requirement model - we are basically speaking about the vision (high level view of req) and the SRS documents(use case diag,detailed use case specs and supplementary requirements).

Analysis,Architecture and Design model-
Analysis is a platform independent model and depends on the SRS ,collaboratin diag -tracked back to use cases and is behaviour specific.

Design model is platform dependent.

Implementation model basically constitutes the actual code ,directories constituting the code.

Now if the project is a enhancement project of the existing application( altogether new functonality is being added)i.e more responsibilities to existing software it is necessary to go back to the prior created UML models and see if it would have any impact on them.As such UML models may have to be re created if the new functionailty is going to have an impact on the existing one

Just want to add on another point, the existing UML diagrams, Collaboration diagrams are also useful to back track from code upto the requirements. If there is an issue comes while testing phase, we need to back track where the requirements came from.

Yes, the existing UML model is still of very much relevance, it represents the BA's understanding of the existing code. It has multiple uses, it is usefull in Acceptance testing, it serves as documentation for any new BA taking over from the person who developed the model. It will serve as a great tool for any further enhancements, changes to the Application.

Should source code be tossed once it is compiled?