Turning down an accepted tenure track for a better one? Nota: This question specifically scopes the issue in an academic context.


Context 
I am currently on the job market for a tenure track. I assert I should get a position in a bottom-of-the-first-tier institution.
Consider I accepted such a position, and that I then get an offer for a top-3 institution. (Obviously, I won't be their first choice, so I should receive their offer after I had to answer to previous lower-range offers.)

Issue
As far as I am aware of the job etiquette in the US, you are not liable to your future company as long as you've signed no contract. So you can eventually change your mind, even if you already shook hands. Of course, you discredit yourself with the given company... but there are plenty of fishes in the sea in the case of industry. This is not the case in the academia, where you will rub shoulders with the colleagues you dumped for a few decades.

Questions


How bad would it be for my reputation/career to turn down a medium-position I accepted for an all-star one, in the case I verbally (phone call or email only, no contract signed) accepted the former ?
Idem, yet with a contract signed (as long as it is permitted in the contract to resign)?
Because of the way the academic schedule works, if you accept a job you are expected to go to that job for an entire academic year.  This is different from other kinds of jobs where hiring happens all year round.  As a part of this, you are expected to withdraw your other applications when you accept a position.  Anything else will usually be seen as unethical behavior by most people.  This can get complicated in some corner cases (eg you take a temporary job but then get offered a permanent job, where the usual ethical move is to delay the start of the permanent job for a year, but often there’s no hard feelings if you just drop the temporary one, especially if you’re nice about it), but it’s the standard ethical rule.