[{plonks:["plonk1","plonk2","plonk3"],_id:1001}]

db.collection.update({},[{"$set":{"plonk":{"$arrayElemAt":["$plonks",-1]},"plonks":{"$slice":["$plonks",{"$subtract":[{"$size":"$plonks"},1]},],},}},])