[{"interactions":[{data:"keep",prompt:"prompt 1"},{other:"keep",prompt:"prompt 2"},{field:"no prompt"}]}]

db.collection.update({},[{$set:{interactions:{$map:{input:"$interactions",as:"interaction",in:{$mergeObjects:["$$interaction",{$cond:{if:{$gt:["$$interaction.prompt",null]},then:{prompts:["$$interaction.prompt"]},else:{}}}]}}}}}],{multi:true})