You really need to stop reading their web site. Lol, just kidding.

The limiting factor for going to higher voltages is the capacitors in the controllers. Most are rated 63v so you are good at 48v battery. I don't know what controller comes with that kit, but you can open the case and look at the caps to see what they are rated at.

Most controllers have a low voltage cutout or LVC. It shuts down the controller when the battery gets low to protect the battery from over discharge. A 36v controller will have the lvc set around 31v. If you just increase the battery to 48v the controller will no longer protect the battery.

You can buy controllers that are rated for higher voltages and that are programmable so you can change the lvc. Edward Lyen sells some very popular controllers over on endless sphere.

48v lithium will be 58.5v fresh off the charger and settle back to 52.8v

Short answer is you should be fine just going to 48v, might check the controller caps. But be sure to get a battery management system (BMS) if you get lithium batteries.