Depends on the school, although I would assume that most will keep your application. In fact, many schools require just an additional letter of recommendation if you are a reapplicant - no secondary essays necessary. If they would keep your secondaries on file, they would definitely keep your AMCAS essays on file. As for whether they would refer back to your old AMCAS essay, I don't think anyone will have a definite answer. If you called the schools you were rejected at and found that your essay was not a problem (and if you still REALLY like your essay), you may want to consider only slightly modifying it to include recent pertinent information. But I'd call the schools on that to make sure it won't be a problem.