[{"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:{$not:{$eq:["$firstName","John"]}},then:new Date(),},{case:{$not:{$eq:["$lastName","Doe"]}},then:new Date(),},],default:"$lastModified",},},},}])