Hi and
to the forum.
If you have this style of external controller:
then your motor and controller should both be able to run on 48V without having to reprogram the controller, so all you need to do is change the battery and disconnect
(or add a suitable resistor into) the Green battery feed wire going to the throttle that powers the LED Battery Gauge.
If you don't disconnect the Green wire, the throttle unit will get very warm due to the 36V battery Gauge running off 48V, and all three of the LEDs will remain brightly lit, even when the 48V battery is almost completely discharged.
The old controllers can be programmed for different voltages, but will still run fine on 24V, 36V or 48V batteries regardless of the programmed voltage setting
(unlike the later vector controllers that will not work until the correct voltage has been set).
Some user with 48V batteries would deliberately set the controller to 24V as it seemed to produce more torque.
Alan