As the big agencies get ready for the biggest week of the year for the biggest advertising medium, changes are coming that can only be called, well, big. The medium is of course broadcast television, which remains a powerful way to peddle products despite the recent inroads made by alternative ways to watch programs, which include the Internet, digital video recorders, cellphones, DVD players and video on demand. Beginning today, the, er, um, big broadcasters will reveal their prime-time lineups for the new season in a week of lavish, star-filled presentations at Manhattan landmarks like Carnegie Hall and Madison Square...