How override Provider in Angular 5 for only one test?

In one of my unit test files, I have to mock several times the same service with different mocks.

import { MyService } from '../services/myservice.service';
import { MockMyService1 } from '../mocks/mockmyservice1';
import { MockMyService2 } from '../mocks/mockmyservice2';
describe('MyComponent', () => {

    beforeEach(async(() => {
        TestBed.configureTestingModule({
        declarations: [
            MyComponent
        ],
        providers: [
            { provide: MyService, useClass: MockMyService1 }
        ]
        })
        .compileComponents();
    }));

    beforeEach(() => {
        fixture = TestBed.createComponent(MapComponent);
        mapComponent = fixture.componentInstance;
        fixture.detectChanges();
    });

    describe('MyFirstTest', () => {
        it('should test with my first mock', () => {
            /**
             * Test with my first mock
             */
        });
    });

    describe('MySecondTest', () => {
        // Here I would like to change { provide: MyService, useClass: MockMyService1 } to { provide: MyService, useClass: MockMyService2 }

        it('should test with my second mock', () => {
            /**
             * Test with my second mock
             */
        });
    });
});

I see that the function overrideProvider exists, but I did not manage to use it in my test. When I use it in a "it", the provider doesn't change. I didn't manage to find an example where this function is called. Could you explain me how to use it properly? Or have you an other method to do that?

Thank you in advance to any one who may be able to help me.