Absolutely be flexible and willing to change plans, cancel ADRs, cancel FPs, etc. Don't let it bother you if everyone (or just one or two in your family) decide they don't like your plan for the day or the next things on your daily touring plan. Don't be afraid to "admit defeat" and leave a park much earlier than planned because people are tired. Don't be afraid to skip rope drop and let people sleep in if they would rather sleep longer. If everyone just wants a QS hot dog instead of the impossible to get TS ADR you have planned, roll with it. If no one is very hungry but you have another impossible to get TS ADR planned, don't feel like you "have to" go and eat anyway just because you were victorious in getting the coveted ADR. Don't be so commando that you forget to stop and take it all in - it is easy to not even look around at the beauty and fun of WDW because you are so focused on getting to the next attraction on time. It's okay to see the ice cream cart and decide ice cream sounds good right now even though it wasn't planned. If your kids decide the resort pool or mini golf sounds like more fun than hitting every major attraction on your carefully planned touring plan - have fun in the pool and playing mini golf instead. Don't be afraid to split up and meet back later if people want to do different things. I once rode the TTA around and around by myself several times because my son wanted to do Space Mountain over and over again and the line was not very long. That has actually ended up being one of my favorite memories at WDW - I realized I had never been alone at WDW and it was awesome to kick back and relax and just take everything in. I am certain we ate at many hard to get TS places that trip and had FPs for all the good rides, yet what I vividly remember from that trip is my time on the TTA.
It is much more fun to just let all the pre-planning go than to have people in your family miserable at WDW because they want to do something different than what was pre-planned. You will see a LOT of moms who have clearly planned for months (or years!) who are yelling, yes YELLING, at the top of their lungs at their husband or kids because they aren't walking fast enough to get to the next thing they just have scheduled, or because they want to do something that "isn't on the plan." You will see kids crying because their mom is mad at them because "they are ruining the entire vacation" because they don't want to ride something they have planned at that exact moment. I can't tell you the number of times I have seen an angrily focused mom barking out orders to her family as she clears a path through the crowds - meanwhile her husband and kids look so sad as they miserably try to keep up with her. It's insane and is one of my biggest pet peeves at WDW.
In the end, just remember that a WDW vacation is supposed to be fun for everyone.
