Migrating field structure in Sanity without losing data

object(node, path, context) {
      if (node._type === 'gallery') {
        return at('images', setIfMissing([]))
      }
    },

Error: sanity.cli.ts does not contain a project identifier ("api.projectId"), which is required for the Sanity CLI to communicate with the Sanity API

import { defineCliConfig } from "sanity/cli";

export default defineCliConfig({
  api: {
    projectId: "<your-project-id>",
    dataset: "production",
  }
});

object(node, path, context) {
  if (node._type === 'gallery') {
    return [at('gallery', unset()), at('gallery', set(node.images))];
  }
},