I wouldn't.
Why? Because there is no reason to me to do so.
I used to own an Iphone 4, I bought it when it came out andit was awesome. That retina display was revolutionary, the overall smoothness and so on. But when the iOS updates started to came out, the Iphone become slower and slower until iOS 7 finally destroyed its performance. It became a complete slideshow, so I decided to get rid of rid and bought an Lumia 920, and I'm satisfied so far.
If there is one thing I love about Windows Phone is the smoothness of the OS, which is crucial for me. Even after the Windows Phone 8.1 Cyan update, my Lumia 920 is ultra smooth, so there is absolutely no reason for me to consider replacing it.
I have to add that in my country (Portugal), there is no such thing as those awesome discounts several US (and other) customers get with contracts, so I have to pay the full price for the Smartphone each time I decide to change.
Regards.