Did anyone have to change their diet cold turkey/go the whole hog/all or nothing for health reasons?

Yes, as I was finally diagnosed as allergic to sulphites, preservatives and salicylate acid. So out went all sausages, wine, jams, dried fruits, a lot of vegetables fruits and spices, coffee and tea pretty much overnight. As a result my skin has cleared and I look younger and healthier, my hair is shiny and my nails grow, my asthma has almost completely gone but unfortunately my weight went up as I was eating stuff I could actually eat without being constantly ill.

I ultimately made a lot of changes to my diet and the way I eat...but it wasn't overnight wholesale 180* changes. I never had anything that would have benefited in the short run from wholesale changes overnight. Most chronic health conditions like diabetes, fatty liver, high blood pressure, high cholesterol, etc are more or less an extension of obesity and lack of movement...that is to say, losing excess weight and getting in regular exercise are going to have a more profound impact on those health conditions than anything else.

My wakeup call came when I was 38 and had my annual physical and came back with triglycerides so high that they couldn't even measure my cholesterol...pre-diabetic blood glucose levels and major hypertension. I was immediately put on medication for my hypertension but beyond that, my doctor basically told me I needed to get my *kitten* together. I had become obese and was very sedentary and he told me that I would get the most benefit by dropping the excess weight and regularly getting some exercise...he told me that improving my diet would help a lot in that improving my diet overall would likely help me lose weight.

The first thing I really focused on was cutting back on calories and getting out for walks everyday. Soda was the easiest place for me to cut calories as I drank about a 6 pack of full sugar Mt. Dew pretty much daily. I started by cutting that back to a couple per day...and then one per day...then one every couple of days...once per week...and then cut it out completely. I dropped about 20 Lbs or so by cutting back and eventually cutting out soda and walking a lot.

I really tried to take the nutritional aspects in chunks and tackle one or two things at a time...I didn't want to overwhelm myself. The next thing was recognizing that I was nowhere close to the daily recommended amounts of fruit and veg...so I made that more of a priority and a goal to at least get somewhere close. The extent of my veggie intake was typically a couple sprigs of asparagus with dinner or a couple broccoli florets...or if I had a big stew or something. I also made efforts to increase my fiber for my cholesterol...started eating oats a few days per week for breakfast as well as preparing more meals with beans and lentils...I started eating more fish and poultry and less fatty cuts of beef.

My exercise eventually went from walking daily to walking most days and jogging three days per week...a few months in and I get back into the weight room after about a 10 year hiatus. I discovered MFP when my exercise activity started to increase and I started doing more intense work and I was curious as to how many calories I was consuming relative to my new more active lifestyle. MFP not only helped in the calorie department, but also in showing where I may be somewhat nutrient deficient or a minimum sub-optimal.

The pounds kept coming off and at the time, I was having blood work and checkups with my doctor every 12 weeks and every appointment showed continued improvement until I was ultimately in the optimal range for everything. That was about 8 years ago and I have more or less maintained all of that except with COVID and whatnot things got a little sloppy this year and my BP started creeping up as well as my cholesterol...so working on that and trying to drop about 10 Lbs or so to get back to my normal maintenance weight.

I'm currently in a transition from the Standard American Diet to a doctor-directed low-protein diet.

PKU is an amino acid metabolism disorder. Amino acids are the 'building blocks of protein'--my body sucks at breaking down one of them so I have to limit my protein from food.
(Why I was ever off the diet is a long story and off-topic, so I'll spare you the details.)

Fun fact: Limit yourself to 10g protein /day (not recommended) and you'll quickly realize that EVERYTHING has protein.

I eat fruits, vegetables, and grains sparingly. That's it. I do eat non-dairy and non-grain products that are generally coconut (fruit) or cassava (vegetable) based--my favorite brands are Daiya, SoDelicious, and Siete.

But because everyone needs protein, most of my "protein" comes from an amino acid supplement formula which goes from as high as 800 calories if you want your formula to taste "good", to as low as 300 calories if you want to hate your life.

(It's not as bad as it sounds. Really it's not.)

I'm quite surprised 2 people have taken time to disagree with my replies. I'd be interested to know why, when I explained that I'm allergic to salicylate and sulphite which is basically leafy green plants or plant derivatives. I can't eat broccoli or strawberries even if I wanted to as my face puffs up, my throat swells and I get mild anaphylaxis, sometimes severe from high levels such as tomato sauces, coconut oil or heavily spiced curries. I have a limited safe food list mainly root veg, bread meat and dairy though I don't find meat very filling so pure keto doesn't suit me as I gain weight quickly when combined with my steroids or am very hungry. I have a good balance for me of simple carbs, animal protein and small amounts of safe fruit such as peeled apples. I wish I could eat plant based but when I was very ill the more traditional healthy food I tried the more sick I became. I couldn't understand why drinking coffee or eating a salad would preclude a severe asthma attack until testing confirmed my rare but well studied allergies.