Just to add my take. The aWatch is unlike any product Apple has ever made and is a small pivot shift for them. I think the aWatch will be upgradable, but not user upgradable. I believe Apple will probably offer a couple of options. The price will be high enough to make it N/A for the Sport. Here is an example of what could be offered. Just wild speculation.
1. Battery $125
2. Battery and S chip $250
3. Battery, S chip and sensor back $350
4. crystal/OLED $150
I hope this is the case.
I wonder if we'll hear more concrete details about the watch in January.