Modify deeply nested data in Apollo client query result

Question

I'm using @apollo/client to fetch data from Storyblok's API.

I want to iterate over all the fields in the returned data to potentially sanitize its content (it contains raw HTML). Because Apollo makes the result data immutable, I'm not able to do this:

const client = new ApolloClient({
  uri: "https://gapi.storyblok.com/v1/api",
  cache: new InMemoryCache(),
  headers: {
    token: "EXAMPLE",
  },
});

function sanitizeFields(obj) {
  for (const field of obj) {
    if (field?.type === "custom-rich-text") {
      field.content = sanitizeContent(field.content); //  This generates an error because I'm trying to modify a read only object
    }

    if (Array.isArray(field) || isPlainObject(field)) {
      sanitizeFields(field);
    }
  }
}

const postData = await client.query({ query: MY_QUERY });

sanitizeFields(postData?.data?.Page);

How can I modify deeply nested data returned by @apollo/client without knowing the particular field name beforehand?