Transferring to a Top-10 PhD program in Theoretical Physics from a Top-50 program in US for better job prospects in Academia?
Well, better is better, in academia just as anywhere else.  Certainly if you got admitted to a Top 10 program and a Top 50 program then all other things being equal you should enroll in the Top 10 program.  (In fact, even things being only mildly unequal, you should enroll in the Top 10 program.  You should seriously consider the Top 50 program only for serious personal considerations -- e.g. if because you are caring for a family member, you cannot leave your hometown -- or because you have a preternaturally good fit with a particular faculty member.)  

But that's not what you're asking; you're asking about transferring later to a top program, which presumably means that you have not yet been admitted to one.  That is not a good plan.  The competition to get into the very top PhD programs is of a roughly similar level of difficulty as the competition to get a post-PhD academic job.  The main difference is that the former competition is more cut-and-dried whereas the latter competition has great amounts of uncertainty and randomness.  In other words, it's actually pretty easy to make most admissions decisions at top ten places: there are certain agreed upon metrics that most or all graduate programs take into account, and the top programs pick from the top applicants according to these metrics.  

This has the following important implication: if you applied to the top programs in year N and didn't get into any, it is very difficult to transfer into them in year N + 1, N+2,...The idea of "transferring from one graduate program to another" is already not very standard, and when it is done it is often done to deal with a problem rather than by virtue of exceptional success.  The one exception is that it is common for one to transfer from a master's program to a PhD program.  However, in many STEM fields in the US, the best students just enroll into a PhD program straightaway: their undergraduate preparation is just as good -- or more often, better -- than what American students graduating with master's degrees get.  (In other parts of the world, a separate master's program may in fact be mandatory, so things are a bit different.)  Most American STEM master's programs that I know of are "PhD lite": they are for students are either not as strong, not as prepared, not as committed, or some combination thereof as the PhD students, so getting a master's degree is -- relative to the desired goal of transferring to a top place -- no great distinction.  

To be able to transfer from a top 50 HEP program to a top 10 one, you would have to do some truly excellent research at the beginning of your career.  Transferring out of a program in which you've had such sterling success is worth some second thoughts, but if you did this work largely or entirely on your own and the continuation of it would be aided by transferring to a top program: OK, do it.  However, this is very rare, and if you do something that great, then you're on your way to success independently of your transfer plans.  

I think that if you spend the first few years in your PhD program with the express goal of transferring out as soon as you can, there is a substantial risk of that showing through as a lack of commitment to your current program, which could really work against you.  


  I mean, is [there] any huge advantage in the academic job market to be at a Top-10 school compared to a Top-50 school ? 


It is an advantage, yes.  The advantage can be overcome by your own work, and if you want a very distinguished post-PhD academic job, then getting a PhD at an absolutely top program with a world-famous advisor is not a golden ticket.  (I know this as well as anyone: feel free to look up my academic past.)  If you go to a top program and do "about average" there, then you will probably not get a top academic job because you will not be getting the top drawer recommendation letter from your famous advisor.  However, that same advisor could help you out getting placed at a lesser institution, or the pedigree of the institution could help (a lot, in some cases) if you wanted to get a non-academic job.  Better is better.


  I have earlier worked with Professors who graduated from such top schools and they say that famous advisors are like "King-Makers" who get their students well placed in academia after PhD since they have influence in the field.


The people that the King-Makers make kings were doing pretty well on their own.  A truly eminent advisor can probably place some of their students in very good positions.  But not all of them.  The important thing to remember is that the quality of your own research -- as perceived by the academic community -- is what will get you or not get you an elite academic job.  If your work is superior, it is superior no matter where you are, and the community will recognize that.  If your work isn't that good but is still strong by the standards of the top institution, then having an eminent advisor really comes in handy.  If your work is not as good as many other students of that advisor, then you are not going to get a big career boost.  In some situations, having an advisor who is still well known and recognized by the community champion you as they have never championed a student before, could work out better.


  Also, is it ethically good to change schools like this ?


I see no ethical problem with changing schools in this way (although as ever there are more and less gracious ways to proceed); leaving a program for a much better one is easy to understand and I think few people will hold it against you.  But as I said above, effecting this kind of change is just very unlikely; 
the real concern is what will happen to you while you are angling for it.  If you are just working that much harder to do your best work: great, and great whether you transfer or not.  But don't make plans or behave in a way which will only payoff if you transfer.  E.g. you might reason that merely doing well in the coursework and qualifying exams is not going to be nearly enough to allow you to transfer to a top program (correct) so that instead you should blow off your coursework while you try to solve the hardest problems in the field (disaster).  Learn to grow where you're planted.