Yet even after the change, green card holders can be turned away at the border at the discretion of Customs and Border Protection officers, as only United States citizens have an irrevocable right to enter the country.

Just last year, the State Department issued 31,804 immigrant visas to people from the seven countries, according to a State Department tally. Thousands more were granted student and other visas last year, and those have also been revoked. The letter excepted only a few classes of visa holders, mainly diplomats and NATO military personnel, leaving au pairs and businesspeople, athletes and artists, even victims of human trafficking without exemptions.

“Basically, they are trapped without some other measures they would have to take, another application process they would have to go through,” said Doris Meissner, senior fellow and director of the United States immigration program at the Migration Policy Institute and a former commissioner of the Immigration and Naturalization Service.

Mr. Trump’s travel ban led to chaos at American airports, left would-be immigrants stranded at airports around the world and shut refugees out of a country that had already issued them visas.

“The gathering of the appropriate officials to actually make this order operational did not even happen until after the order was signed,” Senator Claire McCaskill of Missouri, the top Democrat on the Homeland Security and Governmental Affairs Committee, said in a statement on Wednesday after a closed-door briefing by Homeland Security officials. “No wonder it was chaotic and rocky.”

Ali Hamandi, a Ph.D. candidate at Harvard in health policy, said he had just returned from speaking with university officials about whether the policies could affect him. A Canadian citizen born in Iraq, he said he had found so much contradictory information online that he felt his only option was to cancel his travel plans and stay put.

“There are layers of misinformation,” Mr. Hamandi said. “Emotionally it’s quite devastating.”

In the hours after the executive order was signed, border officials detained or turned away anyone who was born in or had a passport from one of the seven countries, even those who also had citizenship in countries other than the targeted seven. But after the intervention of British and Canadian diplomats, Trump administration officials agreed to exempt dual-passport holders who presented passports from other countries. Under the new rules, dual citizens can now legally re-enter the United States if they leave, officials said.