I'm already looking into a higher capacity battery from Samsung. Same LiFePO4 battery, same voltage, higher capacity. Seems the XR is the same setup with higher capacity batteries and a slightly larger battery box.
I see no reason why a upgrade isn't possible, however this is FM we are talking about, they would rather sell new than a 'upgrade' for the +.
Once the specs on the XR are released, I'll know what the upgrade consists of, my theory: just higher capacity batteries that Samsung released earlier this year. BTW, those newer batteries are only about $5 each!