You can find whether a new ticket was issued (although not why it was reissued) by comparing ticket numbers or looking for a new ticket receipt (with ticket number--not just the PNR or itinerary email)in your email. The delta.dumb website should show you the current receipt (with ticket number), IIRC as a PDF while you should have a receipt from the time the ticket was purchased that shows the original ticket number. IIRC there was a way to click on the ticket number and see the whole history of previous ticket numbers ("exchanged for"), but that functionality might be gone.