We were on this cruise and went to Tuscan Grille two nights and Murano once. The Tuscan Grille was truly a wonderful dining experience. We had Norest as our waiter the first night (2nd day of cruise) and he was amazing. Nothing was ever too much, and his demeanor was extraordinary. My wife tends to want to deviate her order from the norm, and he did it all with a smile, and everything came out exactly as she asked. We asked for him on our second visit (6th day of cruise), and it was the same as our first visit. The truly standout dishes were the lobster rigatoni alfredo and the veal marsala. I had the ribeye both nights and it was nice little steak, cooked perfectly (medium rare) both times.
Now on to your question about reservations. Before the cruise, I bought the 3 individual specialty restaurants separately, and not as a package. I wanted specific times and with the package, you had to wait until on the ship before making your reservations. I think I paid like a total of $20 more pp, to do it this way. We usually like to eat around 6:30 P.M. and didn't want to get stuck with 8:30 P.M. or later. Right before the cruise I checked, and Murano said it was completely booked the entire cruise, while the Tuscan Grill only had late seating (> 8:30 P.M.) available. I don't think it would have been a problem switching our times onboard, though. Every day, right by the entrance to the Oceanview cafe, there is a person selling/pushing specialty dining. Waiting for my wife, a few times, I heard him tell people that for all the specialty restaurants, whatever time you wanted was available. Whether true or not, I'm not sure, since I didn't stick around for the whole sales pitch. But, our first visit, the Tuscan Grill was pretty empty, while the second was quite a bit busier. There were multiple parties of 10+ people. Murano we decided to eat later (8:00 P.M.), since we were in Curacao (day 5) and didn't want to rush back to the ship. When we finished dinner (~9:45), there were only 5 other parties left in the entire restaurant. I don't think I saw anyone come in after 8:30, even though reservations go out to 9:15 P.M. Even at 8:00 P.M., Murano was never more the 75% full. So, after all my meandering, I think you should be fine switching up your dining reservations on board.