First, if you do not think you will be happy at the institution (choice of advisors, environment, etc), then you should not attend that institution. It is not uncommon for students to change their area of interest between the time they apply to grad school and the time they accept (and, also, during the first few years of graduate school!), so some schools that were of interest in December may no longer be as relevant by April. However, you should consider your options very carefully. There is little difference in resources/quality of education/opportunities between the schools that you have been accepted to and those to which you have currently received rejections (I assume that you consider these 5 schools the "top 5" because they did not accept you - in reality, several of the schools you've been accepted to are considered to be in the top 5 for various sub-fields of astronomy and several of the schools that have rejected you are not in the top 5 for many sub-fields of astronomy). All of these programs have significant resources and strengths in many areas. So, unless you are significantly changing research directions such that being part of an astronomy department no longer makes sense (from astronomy to condensed matter, for example), it is not obvious that you will have better options next year.
Second, you need to discuss this decision with your letter writers. They know you best, and their advice is likely to be more relevant than that from strangers on the internet. In addition, if one of my students came to me with this scenario , I might be reluctant to write as strong a letter in the subsequent year if I did not understand fully their reasoning. In other words, for your plan to work, you need to have your letter writers agree with you, and therefore continue to write strong letters. If they think that you are needlessly worrying about prestige, rather than about research opportunities, then you may not even get into a top 10 or top 15 program in the next round.
Finally, if your change in research direction is not as drastic, such that there is significant overlap between astronomy/physics/astrophysics (high energy astrophysics instead of cosmology, for example), then you may want to ask each of the schools you have been accepted to whether or not it is possible to complete your dissertation research under the supervision of faculty in the related department. Most schools are quite flexible in this regard. Once enrolled, you can consider transferring to the other department, but most of the time students just work with the research group that is the best match, regardless of the department of record.