One of the important things to keep in mind when changing to a vegetarian diet is to ensure you get enough protein and iron. The latter especially if you are female.
If vegetarians and vegans have a varied diet (of fruits, vegetables, nuts, seeds, fungi and algae) protein is the last thing they need to worry about as the building blocks of protein (amino-acids) are present on a variety of foods. For example, dark green leafy greens are rich in a wide variety vitamins and minerals including iron and the full range of amino-acids required for our bodies to build protein, a must have in every diet. Think about it, the biggest land mammals eat only greens ; ) On the other hand vegans and veggies should pay particular attention to intake of vitamins B12 and D, iodine and omegas. I've been vegetarian for 6 years and vegan since the beginning of 2013. My partner has been vegan for over 5 years and is in her second year of nutrition studies. We'll happily answer specific queries regarding vegetarian or vegan diets. Or at least point people in the right direction.