That could be a BIG problem. When you go to make the change you have to completely rebook which means that it would have to be available for anybody to book those 8 nights under
free dining in your room type, and to make it worse if you just cancel with the idea that will create the availability it won’t as it does not go directly back into inventory and may never go back in under free dining inventory. You are best off booking the nights you are certain you can be there and then if you need to add a night the worst case is you book it as a one night reservation and take the chance you might have to switch rooms; however if it’s same room type odds are you would not have to move. You would not get the FD for the extra night but that is better then having your entire FD package messed up because of an extra night that you cannot be there.