That is absolutely what happened. You have to remember, Gone in 60 Seconds came out in 1974, so in keeping with the current times, the 1970 Challengers were facelifted to look like a newer challenger. Also, with almost four times the amount of '70s on the street compared to '73-'74s, it was very logical to make that move.
In the first scenes, the Challenger stolen from LAX had it's VIN swapped, with a wrecked challenger bought at auction. If you pause the movie, you will see that a Real Deal 383 R/T was destroyed! I can't remember off the top of my head, but I believe that car was an LA built R/T as well...
Later
KFD