I haven't done this but I'm thinking of it so please critique me.
Keep 2.0 bottles after using them.
Figure out how much 1.8 powder would go into a 2.0 bottle to make the 14 fluid ounces that the 2.0 bottles contain. (Just because a bottle of 2.0 contains 400 calories of 2.0 doesn't necessarily mean 1/5th of a 2000 calorie 1.8 bag would go in the bottle.)
Put the right amount of 1.8 powder into a 2.0 bottle and do this for a bunch of bottles.
When you want a bottle of 1.8, add water to the bottle (and you'd probably have to add water, then shake, then add more water, like you do when you make a pitcher of 1.8).
UPSiDES: You could make the less expensive 1.8 at will at 400 calories at a time instead of making 2000 calories and having to be sure you consume it in 2-3 days.
DOWNSIDES: It would be a bit of a hassle, but the idea is to do a bunch at once and then it's done. And it might be hard to get the powder into the 2.0 bottle hole but I'm thinking a funnel might help with that.
Okay, there it is, what do you think? I doubt that this is such an original idea that nobody has thought of it before.