Ive been looking at this mainly for just when I dont feel like riding a bike, but kind of wish the range was longer

https://www.eastcoastwholesales.com/e...oters_x900.htm
How hard would it be to change to better batteries or add more?

First this is a bicycle forum, so you should find a better forum maybe endless sphere to ask questions about a non-bicycle electric vehicle. Second you need somewhere to put the extra batteries.