Its a bit more complicated than just replacing one's tank or even retrofitting an older 3.5gpf/13lpf toilet to make it into a dual flush toilet. The older bowl hydraulic designs used in the States won't work at the efficiency needed to make it into a true HET. Unfortunately, we have too many early low performance 1.6gpf toilets still in use that require double flushing which negate any intended savings.
To be upfront, I own a company that manufactures & markets sustainable building & water conservation products, including DFTR kits & HET WaterSense toilets. We're proud to be an EPA WaterSense partner & offer a quality product line as a lower-cost alternative to the status-quo.
Whether going with a retrofit kit or replacing one's existing toilet with an HET model, one should expect a minimum average 25% or greater reduction in water consumption. It may not sound like much but it does add up in annual savings, especially when combined with changing out other inefficient fixtures in one's home or business.