Yes, to both questions

. If you upgrade from the beginning and decide to downgrade and go with just the free plan later on, be sure to make the change before the 45 day mark, otherwise you will be charged a $50 change fee. Disney usually waives this fee if the price of your package goes up, so if you decide to upgrade later on, you most likely will not be charged a change fee. Also, the
DDP must be added at least 3 days prior to arrival if not originally booked, so I imagine it would be the same time frame to upgrade a dining plan

.