I had to upgrade a seatpost battery when I changed my di2 setup from 10 to 11 speed. The old one did not have enough memory and I could not update component firmwares to make them work together. Also I had to visit Halters because a regular DI2 changer does not support some firmware updates. Now I'd rather buy a complete di2 kit (including shifters) and sell old stuff.
For those playing along at home: the battery (or battery cradle, on the 'old' external road batteries) is the controller for the Di2 system. It can prevent a user from using the newest components (particularly: d-fly transmitters) on the basis of never-having-been-designed-to-work-together. *edit* This is the same mechanism that lets you use either 10 or 11 speed road or mountain bike derailleurs, but not a mixture of any un-like components. Certain components are considered agnostic, like display units (typically MTB) or D-fly, but will still only function--and allow the system to function--within their respective generation. */edit*
The d-fly transmitter can be a PITA, too, since it was designed to only be updateable using bluetooth (why? WTF knows?), which is a pain for a shop, that has hardwired tools to do so for the rest. I guess the bonus here is that the end-user can update everything through the app, if desired.
BTW: if it's working, and you're not looking for a specific feature, DON'T UPDATE OLD Di2.
