When switching from a lead-acid battery pack to a lithium battery the Voltage should be the same unless you are overvolting the scooter, however, there are other factors to consider also such as the lithium battery's maximum continuous discharge current rating and its open circuit Voltage range/discharge cutoff Voltage.
Lithium batteries have a battery management system (BMS) boards inside of them which control how much current can be demanded from them and how low their Voltage level can be drawn down to. If either of these limits are exceeded the BMS will shut down the battery to prevent damage to it. Usually, the battery will need to be connected to the battery charger to reset itself back to a functioning state.
In the case of an unmodified EcoSmart scooter, the controller has a 30 Amp current limit and the original battery pack has a 30 Amp fuse, so a lithium battery with a 30 Amp or higher maximum continuous discharge current rating should be used with it.
Regarding a lithium battery's open circuit Voltage range, this is the range of Voltage that the BMS will allow the battery to operate between. The lower Voltage number is important because this is the Voltage that the BMS will shut off the battery at when it is reached. Sometimes this is also called the discharge cutoff Voltage. The controller also has a low Voltage cutoff (LVC) level so optimally the battery pack will deeply discharge before the controller reaches its LVC so the full amount of the batteries capacity can be used.
For example, the EcoSmart scooter's controller has an LVC of 31.5 Volts so optimally the lithium batteries discharge cutoff Voltage should be at or above 31.5 Volts otherwise the full capacity of the battery will not be able to be used.
We carry a 36 Volt 30Ah LiFePO4 battery that has a 36 Volt discharge cutoff Voltage so its full capacity can be fully used well before the controller reaches its low Voltage cutoff (LVC) level. However, we have seen other 36 Volt lithium batteries with 30 Volt discharge cutoff Voltage ratings and since the controller would shut down when the battery reaches 31.5 Volts these lithium batteries would never be allowed to become deeply discharged and be used to its maximum capacity.
If the scooter is overvolted to 48 Volts then its motor will run faster and stronger and consume more power from the battery pack if the scooter is driven faster than it was when running at 36 Volts. Overvolting may also overheat the controller and motor depending on how hard the scooter is driven.
A battery indicator gauge can be installed by splicing it into the wiring harness between the on/off power switch and power connector to controller.