Ok, sure, a swift change for the better can be done. As pointed out it could lead to constipation, it could lead to a sudden drop of energy at the inset of the new diet, it could lead to headaches, body aches and a feeling of having a slight cold, it could lead to dry skin.
Or your body could take the major change without batting an eye as it were.
You are aiming to lower the fats and added sugars to your diet - that is a good thing, a really good thing.
Some of the easiest ways of doing this are just the way you prepare your food.
Though we are told that certain cooking oils are ok, while others are not - all cooking oils are fat. Simply put.
One of the things I did when I switched from my fat and sugar diet was to pour a drop or three of oil in the cold pan, then take a paper towel and wipe it all over the pan leaving a very thin coating of oil to 'fry' my foods in. Later on, I switched to non-stick fry pans and I bought a spray pump to spray out a fine mist of oil - I use this mainly in baking.
I drink two to three glasses of milk (Big glasses) a day. We go through four gallons of milk between two people in a single week!
Used to be I was drinking the whole milk - Oh My, the calories in whole milk is all from fat. I switched to non-fat (90 calories per serving) and my Partner switched to 2% - this switch was easy for both of us - He didn't like the nonfat, saying it was too weak.
There are butter flavored powders which are nonfat and have the flavor of butter. I use this in my food preparation, in mashed potatoes, over vegetables, even in things like pancakes and breads that I make.
Sources of fat are:
Oils - all cooking oils are fat
Shortening - I have learned to substitute apple sauce for the shortening in baking.
Lard - I never used the stuff
Bacon Drippings - Ah yes, wonderful stuff.
Mayonaise - it's the egg yolk - the Fat Free Version we get is, in my opinion, just as good.
Nuts, Seeds and Peanut butter - High in fat -I do allow myself some of these every so often, but I am mindful that they are fatty.
Dairy Products - Cheese, butter, cream, whole milk - high in fat - except for the butter, there are fat free and low fat versions of these - I find them to be tasty.
Egg Yolks - I allow myself one egg yolk out of three eggs when I have eggs (I like scrabled eggs) When baking the recipe usually asks for one egg, I use a whole egg since I use the apple sauce or prune puree in exchange for the shortening, margarine, oil.
Today meats are reather fatty due to how they raise animals. Skinless turkey breast is the lowest in fat, followed in a close second by Skinless Chicken Breast.
As for red meat, I cut off the fat and either broil it to where the drippings dripp off, or I Bar-B-Q - animal fats in meat are ok in my books, but then I no longer eat meals of just meat and potatoes
Hamburger has become a familar sight here due to the money situation. I use Hamburger in things like tacos, spaghetti sauce, etc. When I'm making something that will be seasoned with HB, I fry the meat, pour it into a sieve and run drain off the fat into the Dog's Dish (Who is suffering from dry skin due to lack of fat in her diet! - more on that in a minute) Then I run hot water over it, then I fry some more, draining it and running hot water over it again - rinsing it out well. Then I add it to the sauce, or I add seasonings to it.
Be mindful, the body needs some fat - can't getrid of them all. Like I said, I allow myself some nuts, I allow myself red meats with some fat still in them. Every once in the while I will eat Micky D's (Though I suffer from it later)
Fats keep the body well lubed and also keep the skin from drying out.
If you do switch suddenly and you are doing an exercise routine, stop the routine for about a month. This gives the body a change to get used to the new fuels you will be giving it.
There will come a time when when you do eat something high in fat you will feel sluggish, tired and the next day you may have loose stool. This will because your body is not longer used to processing high amounts of fat.
Suddenly dropping your sugar in take will lead to lack of energy, up the carbs like grain goods, rice is a good source of carbs. You may experience mood changes, and do not be surprised if you go through physical withdrawls.
Processed sugar is a horrible thing, a man made chemical which is like a drug to the body, and if you have developed an addiction your body will go through withdrawl symptoms - lack of sleep, or sleepiness - anxiety, maybe even depression - cravings, Mood swings, etc.
As for caffine - Ah the wonder drug of the 20th century - I cut my intake back to 3 cups of Joe in the morning - nobody will take that from me, I refuse to give it up - but then I've been waking up with Joe for 21 years.
I was at one time drinking coffee all the time, morning, afternoon and night. I could drink an expresso before going to bed and not lose a single wink of sleep.
5 years ago I stopeed drinking all that coffee and tea and chocolate stuff, if I have a cup of coffee after dinner, I might as well select a good book that night, I will not get much sleep.
I allow myself certain things, like green teas, my coffee in the morning, etc. I switched to caffine free diet coke a few years ago, now when I drink regular coke I feel sick to my stomach, all that sugar and caffine have an adverse reaction to my system.
When I did the caffine cut back I did it one product at a time. Starting with the switch of coke, then removing those later cups of joe, leaving me with my 2 - 3 cups of joe in the morning.
I know this is a lot more data than you asked for, I have a few more lines to add.
Go to your local library and check out 'body-Stat - resetting the body stat permanmently' it is by a man named DeWitt. I have found that book to be the most informative and helpful on when it came to the swtich from meat and potatoes to healthy eating.
The book covers many of the similar questions you brought up here, and also covers in depth why the body gains weight and why we just do not loose weight when we stop eating as much.
Cheers
David