Hi Sheila and welcome to the Disney Parks Moms Panel!
It can be frustrating when you've taken care of your vacation planning including
Dining Reservations and something happens to create a change! I do believe I have good news for you! I have never experienced having a dining reservation time changed because I decreased the number of people in my dining party. It's a bit more complicated when you are trying to add people to your dining reservation. The restaurant may not have enough room for a larger table which would cause you to have to rebook.
I spoke with a Cast Member to confirm you will not have to change your reservation times if you decrease the size of your dining party. If you are certain this person is no longer attending, you can call (407) WDW-DINE or (407) 939-3463 to adjust your party size. Having the correct number of people on your dining reservation will ensure you avoid any no-show fees. I'll admit we have shown up with a smaller party than we had reserved for and we didn't experience any no-show charges, but I prefer not to take a chance.
Sheila, thank you for visiting us with your question. I hope you'll let us know if we can help you with any additional questions!
Wishing you a magical vacation,
Amy H.