[{"_id":"64e929386a2bff904d86118a","name":"Copies","legacy_id":181},{"_id":"64e929386a2bff904d86118b","name":"Temp","legacy_id":182},{"_id":"64e929386a2bff904d86118c","name":"Test","legacy_id":183}]

db.collection.aggregate([{$replaceRoot:{newRoot:{$arrayToObject:[[{k:"$_id",v:"$legacy_id"}]]}}}])