We don't know yet. I assume no, they'll be the same, or at least they'll add in new items but keep the old ones - the latest update added two new seasonal recipes and two more equinox items, for instance, without replacing any items that were previously obtainable. As for getting holiday items off of others, that's definitely possible, but some of the items will definitely be harder to get than others - Rover's briefcase, for instance, is an item you only get once per player (per year, possibly), and can't reorder or make yourself.