Update existing field to value of another field

main.js

conn = new Mongo()
db = conn.getDB('test')
coll = db.coll
coll.drop()

try {
    print(conn.getDB('admin').runCommand({buildInfo: 1})['version'])

    coll.insert({
        id: 123,
        PreviousRecord: 'old',
        CurrentRecord: 'new',
    })
    print('before updating')
    printjson(coll.findOne())

    coll.update({id: 123}, {'$set': {'PreviousRecord': '$CurrentRecord'}})

    print('after updating')
    printjson(coll.findOne())
} catch(e) {
    printjson(e)
}