[{"id":100500,"field":{"field_name":"some_name","some_other_fields_to_preserve":"test"}}]

db.collection.update({},[{"$set":{"field":{"$objectToArray":"$field"}}},{"$set":{"field":{"$map":{"input":"$field","as":"f","in":{"$cond":{"if":{$eq:["$$f.k","field_name"]},"then":{"k":"$$f.v","v":"some_value"},"else":"$$f"}}}}}},{"$set":{"field":{"$arrayToObject":"$field"}}}])