Database
db={"test":[{_id:ObjectId("63b84e6d5bc77784ee5ecdf6"),demographic:{first_name:"Srikanth",last_name:"R"},change_history:"[{\"change\":\"Gold\"},{\"change\":\"Silver\"}]"},{_id:ObjectId("63b84e6d5bc77784ee5ecdf7"),demographic:{first_name:"Sri",last_name:"K"},change_history:"[{\"change\":\"Gold\"},{\"change\":\"Platinum\"}]"}]}
Query
db.test.update({"change_history":{"$type":"string"}},[{"$set":{"change_history":{"$function":{"body":"function(chgHist) {return JSON.parse(chgHist)}","args":["$change_history"],"lang":"js"}}}}],{"multi":true})