I was originally scheduled to be on a flight from TPA-ATL-BUF on Monday Jan 10. I called and changed my flight to TPA-DTW-BUF. I have an assigned seat for the TPA-DTW portion but I was told my seat for the DTW-BUF would be assigned at the gate. When I went on delta.com to try and select a seat it says none are available. I was wondering if anyone knows what this could mean?? Can I assume this flight is oversold and that I may not get to BUF?? The flight number for the DTW-BUF is 3719.

Thanks.