Yes a sudden shift can be unhealthy
When I was in my early twenties I started quite suddenly experimenting with fasting, juicing, and eventually was a raw vegan for over 2 years. Now 10 years later I am still barely recovering from the shock it caused my system. Are there aspects of these diets that are healthy. Of course. But at the same time there is much evidence that used unwisely they can cause much more damage than the "SAD" diets they are trying to replace. Now keep in mind that I was already eating macrobiotic at times and was not an overnight vegan.
Look at a package of cat food. Many of them say to gradually introduce their product into the cat's diet. Why? Because most organisms (including us) like to approach change gradually. Our digestive system has loads of neurons which are very sensitive to everything we think and do.
My advice? RELAX. Don't be hard on yourself. and don't worry. Just try to normalize your diet to one that you are comfortable with. Try to be happy. Rent a comedy as often as you can. And don't believe the worry warts that there are all these loops you have to jump through to heal yourself. Your body will usually heal itself if we just get out of the way and stop pushing weird routines on it.
And over the next few years try to move towards a healthier diet. But be gentle and don't sweat it if you backslide.