** Apple AAPL.O shares slip 1.2% to $313.61 premarket after KeyBanc downgrades stock to "underweight"

** KeyBanc says higher product prices, weaker U.S. upgrade activity and changing carrier subsidy models could weigh on future iPhone demand

** U.S. wireless carriers are pulling back on device subsidies and promotions, a trend the brokerage expects will lengthen device replacement cycles and reduce upgrade rates

** KeyBanc sees FY27 estimates for Mac, iPad and wearables at risk as demand slows following price increases across product categories

** Forecasts FY27 iPhone revenue growth of 4.9%, below consensus expectations of 8.3%

** Expects Apple's services revenue growth to slow to 7% in FY27, compared with Wall Street expectations of roughly 12%

** Stock recently at 33 times expected earnings, vs average forward PE of 28 over the past five years, according to LSEG data, suggesting it may be overvalued

** AAPL stock has gained about 16.7% YTD, outperforming the Nasdaq's .IXIC 11.3% rise