db={"shops":[{"_id":"...","shop":"pepe","fruits":[{"name":"banana","taste":"sweet"},{"name":"mango","taste":"sweet"}]},{"_id":"....","shop":"dino","fruits":[{"name":"lemon","taste":"tart"}]}]}

db.shops.update({"fruits.taste":{"$exists":true}},[{"$set":{"fruits":{"$map":{"input":"$fruits","as":"fruit","in":{"$mergeObjects":["$$fruit",{"taste":["$$fruit.taste"]}]}}}}}],{"multi":true})