$500?? The iPhone 6+ screen replacement fee is $129.00, and the out of warranty replacement fee is under $400 (well, $399 in Canada, but only $329 in the USA)? If the only thing wrong is the screen, you can ask at your nearest Apple store or authorized repair center for a simple display replacement (and they'll check it out at no charge to make sure there is also no internal damage)?
Also, if you read the warranty, there is nothing at all that guarantees you a new unit for your used unit. Once you were past the unconditional return period, your device became a used device and was never eligible for a new (as in never sold or used) replacement. The only time Apple normally uses a non-factory refurbished unit is when those are not available and thus waiting would delay service. Otherwise, service units are ones that have been refurbished by the original manufacturer back at their plants in China. As Meg says, they appear indistinguishable from new, and do have new displays and new battery as part of the factory refurbishment. Getting one also re-instates the remainder of your original warranty, or 90 days (whichever is greater).