I can answer the question about changing from assigned to flexible dining. We asked to change on Fantasy from Mobile in November and were told we couldn't - but we didn't ask on the 1st day as we weren't aware it was available on this ship.
Daughter and I just came back from cruise on Triumph out of NO on 2/6 and that is the option we were given on this cruise. We booked super late like 1/28 due to a FANTASTIC opportunity and this was only option available. We enjoyed it for the most part but were not aware we would have to wait on occassion. Be aware on elegant night there will be a wait for tables especially around the 6:30-7 time frame - it was about 20 min. We always had a table for 2 and it was never the same table. It was only the 2nd week for this ship to have it and I can say that the service was not up to par but given time to work out bugs I think it would be OK.