I've heard stories about vegans/vegetarians trying to eat meat again after a long period of time of vegetarianism and getting sick. Just an example, one of my best friends talking about his girlfriend rejecting meat after stopping to eat it for 3 months.
Are there any steps a vegetarian/vegan should take before eating meat again? Is there any "transition" time?
Note there are also reasons why a vegetarian may want to eat meat for a short period of time even if they plan to continue to be vegetarian in the long term (for example if they will visit a country where vegetarianism will be difficult, or they have a short term medical condition)
P.S.: This question can be thought of as the reverse of this question.