iPhone 6 Plus. Image source: Apple.

Even as Apple (NASDAQ:AAPL) has enjoyed two blockbuster quarters on the strength of the iPhone 6 and iPhone 6 Plus, there could very well be more where that came from. Consumer Intelligence Research Partners has recently released its latest findings on how Apple is doing, and its figures suggest that less than half of the U.S. iPhone installed base has upgraded to the iPhone 6 or 6 Plus.

Fifty-six million more potential upgraders
More specifically, CIRP estimates that 40% of U.S. iPhone users have upgraded to the latest and greatest models. The researcher also pegs Apple's iPhone installed base in the U.S. at 94 million, meaning that there are still roughly 56 million current iPhone users who are prospective upgrades. That doesn't include any possible Android switchers that Apple may be able to win over.

Source: Consumer Intelligence Research Partners.

The iPhone 6 and 6 Plus are about halfway through the first year's product cycle. Once the newer iPhones are launched later this year, these models will naturally fall in price in accordance with Apple's pricing strategy. The installed base will continue to transition to these newer models, as the chart above shows happens over time.

CIRP's estimates are derived from regular surveys of 500 consumers who recently purchased an Apple product.

Not everyone wants a bigger iPhone... yet
All of this information serves as additional confirmation that consumer preferences continue to shift toward larger smartphones. While the 5.5-inch iPhone 6 Plus simply may be too massive for a lot of consumers, it's very likely that the aging 3.5-inch display found on the iPhone 4s probably won't cut it for much longer. CIRP estimates that there are still 8 million of these models floating around out there domestically.

Even if those iPhone 4s users don't upgrade to a 4.7-inch iPhone 6 or 5.5-inch iPhone 6 Plus, Apple still benefits if those users make any upgrade at all to a newer model. They would certainly be eligible for an upgrade under the traditional subsidy model since it's been well over two years, or they could upgrade through one of the early upgrade or installment plans that all carriers now offer.

Same difference
While CIRP pegs iPhone 6 and 6 Plus penetration in the U.S. at approximately 40%, Tim Cook said on the last earnings conference call that Apple's own estimates are that just 20% of the global active iPhone installed base has upgraded to the 6 or 6 Plus. That's why Cook sees "plenty of upgrade head room" ahead.

The disparity between those figures implies that a lot of upgrade opportunities remain internationally. While consumers in emerging markets are generally less capable of affording Apple's priciest devices like its latest flagship iPhones, Apple continues to woo first-time buyers and pursue affordability initiatives to broaden its reach.

The iPhone 6 and 6 Plus have pushed iPhone unit sales to all-time highs, but the larger smartphones aren't done yet.