I had a ticket reissued about a week ago - same thing. Same flights, just now 006 and not 012 ticket stock. Other reservations are still intact with 012 ticket numbers.

I find it a shame that DL can't even send you an e-ticket receipt. What if you have partner airlines involved and credit your miles to a partner airline? Then the miles don't get credited and the other airline wants to see the e-ticket receipt (like DL often requires when you are missing miles but have a ticket issued by an airline other than DL/NW)? They better get their IT department up to snuff pretty soon.