I was worried about the new FP+ system when it was announced and dreading the "structure" it would create. We took our trip over Spring Break in 2014 relatively early in the FP+ process. When we were planning the web site still had bugs and so on top of trying to get comfortable with the new system I was not even sure if we had done it right. We had 8 people in our family and somehow some of our tickets were on one of my accounts and some on another (I had a DVC and a general Disney login. They are now combined per this trip as it got resolved after about 10 hours over two weeks on the phone with many, many Disney tech people) so for me I had a ton of stress even BEFORE we got to the parks. I would be looking at reservations for a ride and see we have 6 even though we had 8 people but when I tried to add the reservation for the other two it said it was already there even though I could not see it. This got resolved just a month before our trip.
Once there however we had the normal daily flow when you suddenly felt like you just wanted to do something else than what we had scheduled for the afternoon. We used the FP app on the phone or at times in our room for the next day and moved things around. It was painless and seamless. I think it was actually more flexible than the old system because once you had the paper ticket you could not go put them back in the machine and say you changed your mind. It tied up your access for at least a few hours. Yes you could go and get something else and just lose those passes but it had its own problems and we lived with them. I found with FP+ this is the same thing. Is it perfect? Of course not. Nothing so immensely complicated and vast could be, but it is not as bad as I think you are worrying it will be.
On the meal ressies, I'm not sure what to tell you because those are definitely a lot less flexible, making it very hard to change the day of. We just made sure we had a lot of conversations with everyone as we planned and checked and triple checked as the months went by that we did not want to make any changes. The only thing you are scheduling is one table service every day and if you are going to packs based on the magic hours, as we normally do (either because they have them or AVOIDING a park because they HAVE them) it means you are pretty comfortable with the park you selected for a given day, even though it is six months out. I do recall 20 years ago being able to walk to any restaurant and get in, but with the dining plan, which removed SOOOOO much stress for me, you just have more people eating. I think when people had not prepaid for meals they'd be tempted to leave the park early and get McDonald's to save money. In our case, especially when the kids were little, I found the Dining Plan the best thing ever. I used to get so upset when my 5 year old would order something, take two bites, and then say they did not like it or were not hungry. I'd lecture about how much it cost me and how we were wasting money, how they should have ordered something smaller etc. Now that I have already paid for it and if they eat, what they eat, and how they eat does not cost me anything else I am so calm about it it sometimes even scares me!

The added "inflexibility" has created so my "flexibility" that I had not even thought about that I was amazed. The kids have tried foods they would have never ordered before because the meal would have been too expensive to experiment or they were just nervous. Now some of our favorite memories are when our son tried popcorn shrimp for the first time and found it was his favorite food "EVER!!!" and then wanted the to bring him more like it was a buffet. We had an extra meal and it was the end of our trip so we just ordered him another dinner. Or when our daughter tried prime rib at Cinderella's castle and had the time of her life.
I think it will only feel structured if you go in making it that way in your head. The new system allows a lot of flexibility that the old one did not, like using the app.
Sure you can have issues like annie had wasting a ride on redoing the FPs but I think that's pretty rare and really just is dependent on what you allow yourself to do. Would 10 minutes matter on a FP change? Maybe, maybe not. If it's that tight I would pass on the change and enjoy the ride, but that's my way of dealing with it and not what annie chose, so to each his own. Bottom line: psychologist's have said we all hate change. This is what is making you fearful. Flying and airports changed forever after 9/11 and doomsday predections abounded of how it would take forever to travel yet it works just fine. The new FP+ system is the same. Go in with an open mind and I think you will be pleasantly surprised.