On FT we often discuss holding onto PMUs until check-in. The disadvantage to doing so is that the pax continues to hold a Y,B or M ticket at that point. If you complete the upgrade then you hold a Z ticket and Delta should treat you like any other BE pax as regards oversold situations, missed connections etc. . The position of a pax with a Z reservation and Y,B or M ticket seems to be somewhat weaker though I can understand an argument that it shouldn't be so.