I would like to extend the range of a 6-speed Brompton with 12% lower gearing. I am happy with the lower end of the range, handy for the hills I need to tackle, but I find the upper end is not enough when I am on the flat areas. Could the cog for the higher gears be changed into something that would give me what I want? I am not too worried about having a range of gears with a large gap in the middle.

Thanks