[{"arrayName":[{"name":"D","_id":"IDOFD"},{"name":"C","_id":"IDOFC"},{"name":"B","_id":"IDOFB"},{"name":"A","_id":"IDOFA"}]}]

db.collection.update({},[{$set:{arrayName:{$concatArrays:[{$slice:["$arrayName",0,{$add:[{$indexOfArray:["$arrayName._id","IDOFB"]},1]}]},[{"name":"REPLACE 1","_id":"IDOFREPLACE 1"},{"name":"REPLACE 2","_id":"IDOFREPLACE 2"}]]}}}])