How do I change existing string type property into reference type property?

There is an existing collection1 having a property which is an id of another collection2 but the data type is string. How do I convert this property to an objectId type with reference to the other collection2.

Environment: "mongoose": "^8.6.1", node: 20.17.0

const candidateSchema = mongoose.Schema({
  fullName: { type: String, required: true },
  gender: { type: String },
  cvPath: { type: String },
  cvAddedOn: { type: Date },
  email: { type: String },
  area: { type: String },
  city: { type: String },
  state: { type: String },
  mobile1: { type: String },
  //....
})

module.exports = mongoose.model('Candidate', candidateSchema)



const shortlistSchema = mongoose.Schema({
  ourSrNo: { type: Number },
  candId: { type: String },  
    /// this should have been candidate: { type: mongoose.Schema.Types.ObjectId, ref: 'Candidate'},
    /// and the below candidate related fields should have been populated in query
  candSrNo: { type: Number },
  candName: { type: String },
  cvPath: { type: String },
  mobile1: { type: String },
  //....
})

module.exports = mongoose.model('Shortlist', shortlistSchema);

Required: change the existing Shortlist documents having candId property of type string into reference type property

Definitely I will have to change the shortlistSchema to add the below property

candidate: { type: mongoose.Schema.Types.ObjectId, ref: 'Candidate'},

Is the below script correct to update the existing data? Can it be improved by running one command instead of iterating in shortlist collection one by one?

  const shortlists = await Shortlist.find().select('candId').lean().exec();

  for(const s of shortlists){
    const shortlist = await Shortlist.findById(s._id);
    shortlist.candidate = new mongoose.Types.ObjectId(s.candId);
    
    /// delete unwanted old embedded properties
    delete shortlist.candSrNo;
    delete shortlist.candName
    delete shortlist.mobile1
    ///.....

    await shortlist.save();
  }

I do not want to mess up the existing data, hence need help.