"Simply, carriers must respond by ensuring their installment plans and terms are competitive versus those from Apple and double down on differentiation based on network and tariff," said Geoff Blaber, an analyst with CCS Insight.
T-Mobile US was the first carrier to react to Apple's announcement.
It said on Thursday customers could pre-order an iPhone 6S for $20 a month for 18 months and an iPhone 6S Plus for $24 a month with no down payment, starting on Saturday.
Freedom from a lock-down period is also expected to result in increased churn for U.S. carriers, working to the advantage of market disruptors such as Sprint and T-Mobile.
Sprint's CEO, Marcelo Claure, speaking at an annual telecom industry trade show, said Apple had copied his company's "iPhone Forever" program.
The program, rolled out last month, lets customers upgrade to new a iPhone anytime at $22 per month.
It is unlikely Verizon and AT&T will change plans anytime soon since a majority of their subscribers are on corporate or family plans, which typically see less churn, Wells Fargo analyst Jennifer Fritzsche wrote in a note.
Verizon is the No. 1 carrier for iPhones in the United States.