Preview at WWDC likely in June, followed by September launch.
Apple May Replace Some iPhone 6 Plus Models Needing Whole-Device Repairs With iPhone 6s Plus Through March
Apple specifically states that "orders for whole unit service inventory of some iPhone 6 Plus models may be substituted to an iPhone 6s Plus until the end of March 2018." MacRumors confirmed the memo's authenticity with multiple sources, who requested to remain anonymous due to their positions.
Apple doesn't specify which iPhone 6 Plus models are eligible, nor does it provide a reason for the substitution, but it likely relates to Apple's shortage of iPhone 6 Plus replacement batteries until late March to early April.
A previous internal document obtained by MacRumors states that Apple no longer manufactures the iPhone 6 Plus, so it likely has to reboot its production lines to replenish its supply of both the device itself and batteries, which would certainly take time. Apple did not respond to our request for comment.
It's possible that some customers who have visited a Genius Bar recently opted for a whole-device replacement rather than waiting until March or April for a new battery, which may have depleted the little supply of iPhone 6 Plus replacement units that Apple had, but this is only speculation on our part.
Apple and most Apple Authorized Service Providers can replace an iPhone's display, battery, speakers, rear camera, or Taptic Engine individually, while a defective Lightning connector, faulty logic board, and most other repairs are generally eligible for a whole-device replacement, our source said.
Your mileage may vary depending on the exact damage to your iPhone and the results of Apple's diagnostic tests. Apple Support can be reached on the web or by scheduling a Genius Bar appointment. Apple Authorized Service Providers can be searched for on Apple's Find Locations page under Service & Support.