[{"firstName":"John","lastName":"Doe","lastModified":new Date("2021-12-10")},{"firstName":"Steve","lastName":"Doe","lastModified":new Date("2021-12-10")}]

db.collection.aggregate([{$set:{lastModified:{$switch:{branches:[{case:{$ne:["$firstName","John"],},then:new Date(),},{case:{$ne:["$lastName","Doe"],},then:new Date(),},],default:"$lastModified",},},},}])