Mongodb nodejs update using previous value

I have a mongoDB database and have restructured how I store my date/times. However I would like to update any old documents that are not stored in this format.

Using NodeJS how could I use the previous value of a field and update it using its previous value.

I’m looking for something along the lines of:

function modifyDateFormat(oldDate){...}

let filter = {}
let update = {
  $set: {date_time: modifyDateFormat(previousValue)}

db.collection("collection").updateMany(filter, update)