Applicants may request a particular trading symbol, consisting of a maximum of three letters. Although every effort will be made to reserve the symbol requested, there is no assurance that it will be available. Request for a particular symbol should be made as early in the listing process as possible.
That changed over 5 years ago. For example...if you move from "nas" to "nyse"...you can keep your four symbol ticker. Also, if you move from "nas"..to "nyse"..you can keep your three letter symbol ticker.
For what it is worth, it slipped my mind a while back(forgot about it)...but it is a fact.
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