One would argue that you should change your string fixtures to match and verify that the new API response doesn't break anything with your existing API client. Then you change the API client and verify that all the old tests still work as expected.

Better yet is if you keep the old fixtures and the new fixtures and ensure that your API client doesn't suddenly throw errors if the API server downgrades to before the new field was added.

The mocks have the same fixtures, plus a bunch of plumbing you have to sort out with every major and some minor version number upgrades.

You pay a bigger tax on the mocks down the road.