"While this is a good PR move for Apple to resolve the issue, we are concerned it could be a mild headwind for iPhone unit sales if more iPhone users decide to take the offer instead of upgrading to a new device," Barclays said.
Barclays' analyst Mark Moskowitz estimated that a huge chunk — 77 percent of iPhone users — are among those eligible to upgrade the batteries inside their smartphones.
"In our base case scenario, 10% of those 519M users take the $29 offer, and around 30% of them decide not to buy a new iPhone this year. This means around 16M iPhone sales could be at risk, creating ~4% downside to our current revenue estimate for C2018."
Barclays says it remains neutral on Apple stock. It believes Wall Street is "too optimistic about the iPhone X super cycle" but thinks Apple's growth in services and the new tax law are good for the company's business.
Apple said last week that it would begin to replace aging batteries in late January, but subsequently said that it's able to do so immediately.
The company made the decision following backlash after it was revealed that it was purposefully slowing down the processing speed on some iPhones — which meant older iPhones would run apps and software slower — in an effort to maximize battery performance.
Now, with replacement batteries, users will be able to use the phone at its maximum performance levels.