A couple of years ago, I saw a big display at Home Depot for a kit to convert regular toilets to dual flush toilets. It was a lot cheaper than a new dual flush toilet, and I was thinking about buying one and trying it out. I did a little Google research, and found a couple of posts from people saying they'd had trouble installing it; that cooled my ardour a little, and then the display disappeared before I'd bought one... (Maybe it's still tucked away on some shelf, though - I haven't asked for it.)
It sounds as if it's just as well. Today I was at Costco, and found (to my surprise) that they've got two high-efficiency dual flush toilets for sale (at $119 and $79!). (You never know what you're going to find when you walk in the door there; according to an article I read recently, this is part of their marketing strategy...) I did manage to restrain myself from buying a couple of cases of twelve of these new items, even though they were so cheap. I went home instead, and looked them up at the MAP (Maximum Performance) ratings website (http://www.map-testing.com/). (I'll spare you the details about how they do it, but it sounds as if this is the most comprehensive ongoing toilet evaluation project; they test and rate thousands of different models on their effectiveness and their water use.) I have to check on some more details, but so far it does look to me as if one (and maybe both) of these Costco toilets did very well on their tests.
However - to get back to my original point - in the course of visiting the MAP website I also discovered that they've issued a warning about the problems involved in trying to convert old toilets to dual flush. For several reasons, they say that using these kits should be approached "with much caution" at this point. If you're interested in the details, I'm attaching their statement...