Film Clones & Doubles
http://doppels.proboards.com she was replaced by 2004. As a matter of fact, it probably happened by late 2003.
*Rachel was on the Canadian tv show Slings and Arrows
which debuted on November 3, 2003. She starred in 6 episodes which aired that
fall. She did not appear on any episodes in all of 2004.
*Rachel was in the 2004 film Mean Girls, which was released on April 30, 2004. Yet we see her double's first appearance in the film The Notebook, which was released on June 25, 2004.
So therefore giving production times, it had to be mid to late 2003 when Rachel was replaced. It can take a year or longer for a movie to be released after it is filmed and because the first appearance of "Fachel" is in The Notebook released in June of 2004, she had to have been replaced in 2003 some time. There was possibly an overlap of Rachel and her double in 2003 but it all depends on how far in advanced the show Slings and Arrows was shot and the movie The Notebook was shot.
*It bears mentioning that Fachel made an appearance on the show Slings and Arrows in June of 2005 as a "special guest appearance".
There are a few things I notice right away between Rachel McAdams and her double. Her double has a more prominent forehead for one and a narrower facial strucutre. Plus the voice is different. Look for these differences in the following yt videos:
Rachel in Slings and Arrows beginning at 2:03:
Fachel making her guest appearance in 2005 on Slings and Arrows beginning at 7:40. Notice the different voice:
Notice how the fake Rachel has a more prominent forehead along with a narrower face/jaw. The real Rachel's eyes are further apart than Fachel's.
Also notice I included a pic of Fachel with the blonde hair to show that the hair color isn't the reason for the difference in the look of the two women.