Having a 1.6gpf Drake CST744S, can it be converted to a 1.28gpf easily, or must the entire toilet be replaced?
Well, that's actually an interesting question. The answer is it depends upon what bowl came with it. A couple of years ago, Toto stopped manufacturing the original Drake 1.6gpf bowls (i.e. designed to flush on 1.6 and equipped to hold a certain amount of water in the bowl on refill). The model number of the bowl that came with the 1.6 original elongated Drake was C744S, for example. Now, all Toto Original Drakes come with bowls that are designed to work well on 1.28gpf. Model C744E, for example. The only difference now if you order a CST744E instead of a CST744S is the tank that comes with the now-standard Eco (E) bowl. ST743E for the 1.28, and ST743S for the 1.6. One does a cycle of 1.6 gallons each time you flush, 1.28 for the other. But regardless of how much water is used in the cycle, it goes through the same Eco bowl.
If you have the E bowl, arguably all you would have to do with the S tank is install either the E flapper or a Korky adjustable 3" flapper, and dial it down so it closes sooner. To really get it perfect, you could get a Korky 528MP fill valve, and adjust the refill by dialing it down as well. However, you have no real point of reference, unless you want to measure the water depth in the tank of a properly-adjusted 1.28gpf Drake. One other option would be to buy the Toto TSU99A.X (which is a 528MP with a little gizmo that lets you precisely adjust the refill percentage). You would use that as the reference point, and adjust the flapper so that it drained just enough water so that the time alloted to refill would fill the bowl just to the point that it filled to its natural settling point.
I'm guessing, but not sure, that if you have an S bowl, it probably takes a little more water to refill. And given that the S toilets run at 40 percent refill ratio, which is basically the max you are going to get from any fill valve, to fill the bowl properly you have to run the refill long enough at that max rate that you're probably going to burn 1.6gpf. You might be able to back off the refill time a smidge by cutting the amount of water used in the flush with a Korky adjustable flapper, but it's going to affect performance because one thing you want to start with is a bowl filled to its natural settle level, and the Korky is designed to use a certain amount of water in the flush. Will that reduction in performance be meaningful on most cycles? Probably not, but whether that is an issue for you is really up to how you use it...
Hope this helps.