It really wouldn't be worth it. It wouldn't only be the battery, some £300 upwards these days for a good one, but the higher voltage would require a new controller as well. Since on production bikes some of the bikes wiring is often integrated with that, it could all get complicated as well as expensive.
You'd be surprised how little is gained by higher power bikes anyway. The speed is the same, restricted to 15 mph, but you get faster hill climbing and sometimes better steep hill capability. The fact is that e-bikes are not performance machines, not even electric bikes, just electric assist bikes, and that will soon be reinforced when the pedelec only law comes in, meaning power only when pedalling.
Best to try a few when you get the opportunities before going in for expensive upgrades. If you can get to Presteigne next weekend you'll have the opportunity to try out all the leading machines.
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