Perplexed? You’re not alone. Several people contacted The New York Times about the change. Notices emailed to Equifax subscribers do explain the lock issue, if readers click through to a question-and-answer page. But some subscribers and consumer advocates worry that people may not absorb the details and remain unaware that they have to relock their Equifax credit report. Credit locks are marketed by credit bureaus as a convenient way to prevent lenders and others from obtaining your credit report without your knowledge.

“There’s an opportunity for consumers to be confused,” said Paul Stephens, director of policy and advocacy with Privacy Rights Clearinghouse, a nonprofit consumer advocacy group. If consumers don’t relock their files — and if they haven’t used another option, known as a security freeze — “they’re still going to be very vulnerable,” he said.

Christina Tetreault, a staff attorney at Consumers Union who specializes in financial services, said the situation illustrated why the best way to guard against account fraud is to place a security freeze — not a lock — on your files at all three credit bureaus.

Federal law now requires credit bureaus to offer security freezes free of charge, and they’re much simpler to set up than in the past, Ms. Tetreault said. A freeze accomplishes the same thing as a lock, she said, but with a freeze, your rights are defined by law. With credit locks, she said, the bureaus set the rules.

“With a freeze,” she said, “the power belongs to the consumer.”

Equifax initially told at least some subscribers that it would extend free credit monitoring through TrustedID Premier, according to emails shared with The Times. But later, it switched to an offer of a second year through the credit bureau Experian, a competitor. Some subscribers may already have switched, but the deadline to enroll for the free extra year is Jan. 31, according to a notice Equifax sent to subscribers. (Experian normally charges a monthly fee for its service, called IDnotify.)