How would I parse lambda expressions in C# with ANTLR?

Question

Using this grammar, I need to parse a file with C# code and make changes only to lambda expressions, without changing the rest of the code. How would I accomplish this? Is there a way to do it with a simpler grammar? e.g.

Action actionSingleLineNoImmediateAssignment = null;
actionSingleLineNoImmediateAssignment = () => Console.WriteLine("Non-returning lambda");

into

Action actionSingleLineNoImmediateAssignment = delegate () { Console.WriteLine("Non-returning lambda"); };