WASHINGTON — Prognosticators got a lot wrong about the Washington Nationals. The latest error: The Nats no longer expect to be pure sellers at this year’s trade deadline.In a twist that would have been improbable at the start of the season, the Nationals also plan to acquire some big-league help, according to a person in the organization who was granted anonymity in exchange for candor. Those additions may be light in scope and could be outweighed by what the Nationals also sell — they still have an eye on their long-term plan, after all — but the intent to make additions of any kind is notable.It’s late July and the Nats remain far better than prognosticators expected on Opening Day. Their postseason odds, while slim, are not zero. As such, the Nationals plan to add at least one big-league player and will even consider acquiring a player on an expiring contract if they deem it is the right fit. That would make it easier for them to trade lesser-valued prospects in the farm.This all, of course, is barring a massive collapse before Aug. 3.Rival evaluators still fully expect the Nats to deal from their big-league roster, too, with All-Stars CJ Abrams and Foster Griffin among those who will garner interest. But potentially adding players as well represents a pivot for a team that has seemingly convinced the front office that it is worthy of some level of investment.“To be in a position right now where no one really knows if we’re going to be buyers or sellers is a good thing,” manager Blake Butera said earlier this week. “We have an opportunity to change what we’re going to be doing, or we have an opportunity to win games and put ourselves in a position to make a playoff run. That’s what we want to do.”