[{"_id":ObjectId("5e2bcb2fdc95d08ab83819f1"),"one":"aaa","two":"bbb","three":"aba"},{"_id":ObjectId("5e2bcb2fdc95d08ab83819f2"),"one":"dd","two":"cc"}]

db.collection.aggregate([{$addFields:{data:{$arrayToObject:{$map:{input:{$objectToArray:"$$ROOT"},as:"each",in:{$cond:[{$eq:["$$each.k","_id"]},"$$each",{k:"$$each.k",v:{$toUpper:"$$each.v"}}]}}}}}},{$replaceRoot:{newRoot:"$data"}}])