The "rules" about three-letter calls are not codified anywhere in Part 73, or elsewhere. They're handled in an informal, unwritten way by the callsign desk at the Media Bureau...but here's what I've teased out from following their precedent over the years:
Three-letter calls cannot be moved from market to market. If CC wants to move WOR, it can move WOR to another station in the New York market, but not elsewhere.
Three-letter base calls can be reused in other services, but only in the same market. 105.1 (or 100.3, or 104.3, or whatever) could become WOR-FM. 105.9 in Hartford could not.
Three-letter calls cannot be transferred to an unrelated owner. While CBS could allow some operator elsewhere to to put a "WINS-FM" on the air in a different market (or even in NYC), Clear Channel won't be able to allow a "WOR-FM" or "WOR-TV" under other ownership elsewhere. (However: an owner with three-letter calls across multiple services can sell them to different owners and allow them to keep the calls, so CBS could keep WJZ-TV in Baltimore and sell WJZ(AM) and WJZ-FM to different owners without a forced call change. This of course is a change in the rules from the days when WOR-TV had to change calls when ownership split from WOR AM).
Three-letter calls can be revived after being dropped...sometimes. It's very much at the whim of the callsign desk staffers to decide whether to grant a request. They've granted requests in the past (KHJ, KUT, KRE) and denied requests in the past. They are not governed by any black-and-white rule, just years of precedent and whatever leverage any given owner's DC lawyer can bring to bear. As a general rule of thumb, the FCC seems loath to restore a three-letter call to someone other than the owner/station who gave it up. KUT is an exception, sort of.