The only time I've had difficulty with a food, it was going from being a longterm vegan to eating eggs--gave me a terribly upset stomach (though eating the eggy meal was perfectly enjoyable before it really hit my digestive system). Vegan -> lacto-vegetarian and lacto-ovo-vegetarian -> omnivore were both no problem.

(And conversely, lacto-ovo-vegetarian -> vegan and omnivore -> vegan gave me no trouble either.)