It gets somewhat expensive to go from the 20 mph you probably have now to the 24 mph possible with 48V, First you need to upgrade to a 48V battery, which is about $300-400.. Then you should replace the 35V controller and it display for a 48V model. That is $100-150,
That's like half the original cost of the bike. I don't think that's worth it for a little more speed.
In my case. I had converted a few bikes before I bought the Ecotric 36V fat folder, so I had other batteries and spare electrical available,