Lets say you book the 10th to the 18th on a package deal but then need to either arrive on the 11th or leave on the 17th.....is it easy to modify the reservation or do they charge you?
They will only charge you a change fee if you make the change once you are inside the 45 day window. If you make the change earlier than that, you're ok.
When you add something to a reservation, you are spending more money. So they won't usually charge the change fee. If you reduce your total amount instead, you'll see that $50 change fee sure as you're born.
As long as the room is available, it easy enough to add a day to the reservation. Don't wait until close to the trip if possible because they might not have any rooms available. Just note that is you change the arrival date, make sure the date still qualifies for any package discounts you might have (e.g., free dining).
I don't want to try to add a day later because I know the room may not be available. So I booked the longer trip, hopefully we can stay all the night I booked. Right now it is room only so there would be no penalty. But if FD comes out I need a package.