If you use a shunt that is lower in value than the original, then you can read any current you like with it.
But you then must manually calculate the difference between the original shunt and the new one, and use that to determine the correct readings because they will be off by that factor.
So the best bet is to use a shunt that is 1/10th of the original, and then all you need do is mentally move the decimal point for all current and watt readings over, for them to be close enough to correct.
However, the accuracy will probably be much lower at the low end of the scale, so things like Ah and Wh will probably be significantly off if most of your usage is at that low end.
If you mostly use it at the high current levels, it should be close enough to work.
The Cycle Analyst from Grin Tech at http://ebikes.ca is designed to work with an external shunt and has stuff in it's setup menus to ensure it is accurate with those higher current applications, so it's probably a better choice if you need accuracy and are going to use an external shunt anyway. Just be aware that it could take a while for them to ship anything once you order it, depending on personnel availability.