WSET Lynchburg, VA, ABC's oldest affiliate south of DC (1953 start
date) was originally WLVA. And in DC itself, there's WMAL, which became
WJLA; and WOIC, which became WTOP, then WDVM, and now WUSA, and
it's been with CBS since 1949.
WXII Winston-Salem (NBC) started as WSJS in 1953 and changed to the
present call letters in 1972. Sister station WFBC in Greenville, SC (also
1953 start date) became WYFF in 1983; it's been with NBC all 59 years.
WCTI New Bern, NC (ABC) began in 1963 as WNBE, changing to its present
call letters in either 1969 or '70; it has not changed affiliations since it first
signed on.
Neither has WLTX Columbia, SC (CBS), which began in 1953 as WNOK.
WAPI (full-time NBC since 1970) and WBMG (full-time CBS for the same period),
Birmingham, AL, are now WVTM and WIAT, respectively.
KXAS Fort Worth-Dallas (NBC) signed on in 1948 as WBAP (although I think
it shared ABC with WFAA until the late '50s).
KMOV St. Louis (CBS) changed its call letters from KMOX when CBS had to decide
whether to keep the radio (KMOX) station or the television one. They kept the
radio one. Although not a CBS o&o, KCMO television, Kansas City, changed its call letters to
KCTV when the FCC forced the breakup of the radio-television duopoly there.
All the Nashville stations have undergone call-letter changes even though they have been
with their respective networks since the '50s and one, WKRN, even went through a channel
swap with the PBS station:
ABC: WSIX Ch. 8 to WNGE Ch. 2 to WKRN Ch. 2
CBS: WLAC to WTVF Ch. 5
NBC: WSM to WSMV Ch. 4
PBS: WDCN Ch. 2 to WDCN Ch. 8 to WNPT Ch. 8
And dare we forget WFIL/6 Philadelphia, one of ABC's pioneering affiliates, which became
WPVI around 1972? Or WLBW Miami, an ABC affiliate since the late '50s, which became
WPLG around the same time? Or WDBO (CBS in Orlando since 1953), which became WCPX,
then WKMG? Or Orlando's ABC affiliate WFTV, which began in 1958 as WLOF and changed
calls in 1963?