[{_id:1,items:{cheese:{version:3},egg:{version:23}}},{_id:2,items:{cheese:{version:3},egg:{version:23},sausages:{version:3}}}]

db.collection.update({/** your query here*/},[{"$set":{"items":{"$cond":{"if":{"$eq":[{"$ifNull":["$items.sausages",null]},null]},"then":"$items","else":{"$mergeObjects":["$items",{"sausages":{"version":4}}]}}}}}],{multi:true})