Database
[{"_id":ObjectId("60471bd482c0da3c0e70d26f"),"owner":"John","propAvailable":{"val1":true}},{"_id":ObjectId("60471bd482c0da3c0e76523f"),"owner":"Matt","propAvailable":{"val1":{"val2":true}}}]
Query
db.collection.update({owner:"John"},[{$set:{"propAvailable.val1":{$mergeObjects:[{$cond:[{$eq:["object",{$type:"$propAvailable.val1"}]},"$propAvailable.val1",{}]},{val2:true}]}}},],{"multi":true,"upsert":false})