The three biggest companies that collect and disseminate credit information on more than 200 million Americans will change the way they handle errors and list unpaid medical bills as part of the broadest industry overhaul in more than a decade.

Under an agreement announced Monday with New York state, Equifax Information Services LLC, Experian Information Solutions Inc. and TransUnion LLC will be more proactive in resolving disputes over information contained in credit reports—a process federal watchdogs and consumer advocates have long decried as being stacked against individuals.

Most changes will be implemented nationally and will kick in over the next six to 39 months.

The credit-reporting firms will be required to use trained employees to review the documentation consumers submit when they believe there is an error in their files. If a creditor says its information is correct, an employee at the credit-reporting firm must still look into it and resolve the dispute.

Lenders, credit-card issuers and collection agencies report consumers’ debts, balances, late payments and other credit-related information, such as bankruptcies and foreclosures, to the three companies. The data are added to consumers’ reports and are used to calculate their credit scores.