freeze reference DGN

Hi All,


                I have an problem to freeze the reference.  I am working in project wise environment when I am going to create a new version of DGN files at that time attached reference DGN are not freeze with old version DGN.Means if any one update the reference DGN so the attached reference DGN will update in old version as well in new version of DGN. Is it possible to freeze the reference without merging the drawings or when we are creating a new version at that time all attach references DGN will create new version automatically.