Obvious Second Choice

Throughout the history of commercial arts, creators are limited by what's available and what they can afford. Sometimes the artist will be forced to go another direction. Many do it gracefully, others leave so much of their original idea in the piece that what remains can only be seen as a poor substitute.

This trope can apply to Actors, licensed Music, and anything that could have strings attached. Similar issues occur in the tropes Absentee Actor, Captain Ersatz, and Home Version Soundtrack Replacement.

Film

 * Star Trek Generations's producers hoped to get DeForest Kelley to appear as Dr. McCoy, but due to his declining health, he could not get on-set insurance. Most of his lines in the opening scene were given to Pavel Chekov, which is why Chekov drafts reporters as nurses and heads to Sickbay. Scotty was given one line, asking Kirk if he wanted a tranquilizer. (see here)
 * Likewise, they wanted Leonard Nimoy to play Spock, but when he declined his lines were given to Scotty.
 * A music variant occurs in the final scenes of The Devil's Advocate, Evidently the filmmakers wanted to end the film with "Sympathy For The Devil" By The Rolling Stones. It would seen they had to settle for less appropriate "Paint It Black".
 * A gender switch variant in Ballistic: Ecks vs. Sever. The part of Sever was originally supposed to be male, but after Lucy Liu was cast in the role the script wasn't changed to reflect this. That's why there's no UST between her and Ecks (Antonio Banderas) the way you'd expect (as noted on the film's YMMV page). Source: IMDb Trivia.

Live Action TV

 * Get Smart. Don Adams was unavailable for the episode "Ice Station Sigfried" so Maxwell Smart's place was taken by a CIA agent named Quigly, played by Adams' friend Bill Dana. You can tell that very little rewriting was done to the script, and the writers simply gave Maxwell Smart's lines to Quigly. Source: Absentee Actor.

Web Original
"Snob: Oh yeah, Gene Kelly's second most popular song! Let me guess, they're going to replace Beethoven with "On Top of Spaghetti"!"
 * Lampshaded in The Cinema Snob's review of the A Clockwork Orange porn parody A Clockwork Orgy, which replaces "Singing In The Rain" with "Take Me Out To The Ballgame":