I would presume since nothing is specifically said about an expanded format, that there is none. However more details are sure to be coming, so this is just speculation right now.
Also, day 1 of Nats for TCG is 6-9 rounds swiss, day 2 is remaining swiss and then top 8, possibly top 4 if time allows, then day 3 is the remainder. According to this schedule, I see no time to switch a deck from standard to expanded, while still keeping the event running. Remember, if there is a change, then those decks and lists need to be checked, which would cause a break in schedule/play Saturday, and making it so that only the semifinals and finals are expanded is also ludicrious.