On the Lafree Twist I had, gearing up to raise the maximum assisted point from 15.5 mph to 17 mph reduced the range on it's 6.5 Ah battery from 20 miles to 17 miles roughly.
It's not possible to predict with absolute accuracy. Current will be used at every band of power applied for longer than before at exactly the amount by which you increased the assistance cutoff point.
Conversely, if you reduce the cutoff point, it's possible you would increase the range, but that depends on whether you pedal at speeds beyond the cutoff point of course. If you didn't, you could actually again increase consumption by a proportion of the increase in journey time.
One member has initially reported little apparent difference on his Agattu with a sprocket change, and I think as long as the change is kept moderate, say to a 19 tooth or possibly 18 tooth sprocket, the range will be affected very little for a fit cyclist. The worst effect would be in gearing for 20 mph or more, since the bike would be permanently using current, and could be spending much more time in the higher power bands to maintain the performance.
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