Channel Hop: From ABC to ESPN. As both channels are owned by Disney, this is only a jump from broadcast to cable.
Practically, however, it was a wholesale change, as much of ABC's old production crew (including the entire commentary team and the lead producer and director) were graciously allowed to leave for NBC, while ESPN's old Sunday Night Football crew formed the backbone of the new MNF production.
Follow the Leader: Sunday Night Football, introduced in 1987 and variously aired on ESPN, TNT, and (since 2006) NBC, was pretty much expressly created to be an equivalent to MNF one night earlier. More recently, Thursday Night Football on NFL Network (and, later, CBS/NBC) has similarly expanded on the concept.
The Pete Best: Keith Jackson. When Frank Gifford was unavailable during the first season, executive producer Roone Arledge hired Jackson for the play-by-play. When Gifford became available, Jackson was unceremoniously fired. According to the book Monday Night Mayhem, Jackson didn't even know he'd been dumped until media reporters called him, asking for comment.
In contrast with many Pete Bests, however, things ended up working out for Jackson, who went on to a long and prominent career as ABC's lead college football voice.
NFL Commissioner Pete Rozelle originally wanted CBS to get the Monday night package; he gave them right of first refusal, only for CBS founder William Paley to nix the package due to potentially disrupting their strong Monday night lineup. NBC declined due to airing movies on Monday night; allowing for last-place network ABC to get the prime-time lineup.
Such well-known sports announcers as Curt Gowdy, Vin Scully, and Jack Buck were all offered the MNF play-by-play role prior to the first season, and all turned it down for various reasons. (Buck did, however, end up calling Monday night games for many years...on CBS Radio.)