If Delta is busy, as they have been the last few weeks, it may take a while for the tickets to get issued. They go by date order of your departure. Every once in a while, a PNR will fall through the cracks and not get reissued, which results in big problems at the airport as airport personnel are not generally trained to reissue mileage upgrades. I would give it some time and put it on your calendar to check with Delta in late April. If it is not done by then, you should ask for the reissue desk and have them pull the PNR and reissue it while you are waiting. It may take a little time. The flights are rebooked in upgrade class, accordingly to what you indicated, so you are not in any danger of losing the space.