Runtime method modification

Question

I have the following code, using Mono.Cecil:

        {
            ModuleDefinition module = ModuleDefinition.ReadModule("library.dll");
            TypeDefinition type1 = module.Types.Where(t => "Namespace.Destination".Equals(t.FullName)).Single();
            TypeDefinition type2 = module.Types.Where(t => "Namespace.Target".Equals(t.FullName)).Single();
            MethodDefinition method1 = type1.Methods.Where(m => "Test".Equals(m.Name)).Single();
            MethodDefinition method2 = type2.Methods.Where(m => "Test".Equals(m.Name)).Single();

            var processor = methodTesta1.Body.GetILProcessor();
            var newInstruction = processor.Create(OpCodes.Call, methodTesta2);
            var firstInstruction = methodTesta1.Body.Instructions[0];
            processor.Replace(firstInstruction, newInstruction);
        }
        
        namespace Namespace
        {
            public class Destination
            {
                public String Test()
                {
                    Console.Write("Destination method");
                }
            }

            public class Target
            {
                public String Test()
                {
                    Console.Write("Target Method");
                }
            }
        }

I'd not like to create a new "dll" file or overwrite the current, I want to modify class only at runtime. How can I "persist" the modification and create a new instance of Destination class with modified method? Is there a way to do it?

EDIT: The objective is execute a different method body, when certain method is called, which return a certain type.