How to replace an image referenced hundreds of times in Sanity?

import {defineMigration, at, set} from 'sanity/migrate'

const OLD_ASSET_ID = 'image-abc123...' // Your old image asset ID
const NEW_ASSET_ID = 'image-xyz789...' // Your new image asset ID

export default defineMigration({
  title: 'Replace image asset across all documents',
  
  migrate: {
    document(doc, context) {
      const mutations = []
      
      // Helper function to recursively find and replace asset references
      function findAndReplaceAsset(obj: any, path: string[] = []) {
        if (!obj || typeof obj !== 'object') return
        
        // Check if this is an asset reference
        if (obj._ref === OLD_ASSET_ID) {
          mutations.push(at([...path, '_ref'], set(NEW_ASSET_ID)))
        }
        
        // Recursively check nested objects and arrays
        for (const [key, value] of Object.entries(obj)) {
          if (key.startsWith('_')) continue // Skip system fields
          findAndReplaceAsset(value, [...path, key])
        }
      }
      
      findAndReplaceAsset(doc)
      return mutations
    }
  }
})

export default defineMigration({
  title: 'Replace specific image field',
  documentTypes: ['post', 'page'], // Only run on certain document types
  
  migrate: {
    document(doc, context) {
      const mutations = []
      
      // Check mainImage field
      if (doc.mainImage?.asset?._ref === OLD_ASSET_ID) {
        mutations.push(at('mainImage.asset._ref', set(NEW_ASSET_ID)))
      }
      
      // Check thumbnail field
      if (doc.thumbnail?.asset?._ref === OLD_ASSET_ID) {
        mutations.push(at('thumbnail.asset._ref', set(NEW_ASSET_ID)))
      }
      
      return mutations
    }
  }
})