metamask-extension/app/scripts/migrations/005.js

const version = 5

/*

This migration moves state from the flat state trie into KeyringController substate

*/

const extend = require('xtend')

module.exports = {
  version,  

  migrate: function (versionedData) {
    versionedData.meta.version = version
    try {
      const state = versionedData.data
      const newState = selectSubstateForKeyringController(state)
      versionedData.data = newState
    } catch (err) {
      console.warn('MetaMask Migration #5' + err.stack)
    }
    return Promise.resolve(versionedData)
  },
}

function selectSubstateForKeyringController (state) {
  const config = state.config
  const newState = extend(state, {
    KeyringController: {
      vault: state.vault,
      selectedAccount: config.selectedAccount,
      walletNicknames: state.walletNicknames,
    },
  })
  delete newState.vault
  delete newState.walletNicknames
  delete newState.config.selectedAccount

  return newState
}