if you are thinking of changing one meal per day, see if you can change your breakfast. eating more protein, less carbs, less sugars will give you more energy. you might be setting yourself up for very low energy just with eating cornflakes for breakfast.
one easy way is to make egg cups--mix eggs, grated cheese, some ham and / or veggies, and bake those, in a big batch in muffin trays. you can freeze them for a week or so. then you just grab one or two for breakfast, and warm them up.
I find simple stews are even easier than stirfries, especially if you have a delayed start option on your stove. Toss veggies and some meats together in a baking pan, put it in the oven for a couple of hours, and your meal is ready by itself.
when i was in my tired /low energy phase, i bought baby potatoes, and baby carrots so that i didn't have to wash or cut anything at all.
if you can afford it, buy some pre-cut trays of veggies--easy to snack on. nuts are great for snacking as well.
as for long term effects, you are changing your blood chemistry. your cholesterol could be going up; you might be making yourself sensitive to gluten. and yes, of course, diabetes, as a pp mentioned. most people have the tendency to only start taking care of their health when something really bad happens. maybe find a diabetes forum, or chron disease forum, for an immersion experience of what it is really like to live with diabetes. maybe this will help.