[{"_id":ObjectId("60471bd482c0da3c0e70d26f"),"owner":"John","propAvailable":{"val1":true}},{"_id":ObjectId("60471bd482c0da3c0e76523f"),"owner":"Matt","propAvailable":{"val1":{thisfieldwillbedeleted:1,"val2":true}}}]

db.collection.update({},{$set:{"propAvailable.val1":{val2:true}}},{multi:true})