Other way around. That IS Natasha, and Scarlet Johanssen's character is Yelena, though that's yet to be revealed.
Reason #1: Yelena spent some time impersonating Natasha, getting plastic surgery and dying her blonde hair red in the comics. She starts out as her own character, but later becomes the faux Natasha. This would explain why ScarJo is referred to as "Natasha" even though that's not true.
Reason #2: ScarJo's "Natasha" gets chewed out by Cap in Winter Soldier for nearly compromising the Batroc mission. Similarly, Natasha has scolded Yelena for treating espionage like a game. ScarJo's character is far more gender-stereotyped and inexperienced compared to what audiences would think of regarding an ex-Soviet super spy, while Bridget Regan's character is extremely convincing by comparison, playing the part of "Dottie Underwood" very well while pulling off a more convincing assassin persona in private. I never saw ScarJo as a convincing Widow, but I'd accept it if she was really Yelena.
Yelena also had past ties to Hydra.
Reason #3: Mink's automatic revolver seems as though it would become the basis for Natasha's trademark gauntlets. Natasha was the first Black Widow, being the top of her class at the Red Room, and Yelena was the second, so while there were many girls in the program, there is no evidence of other Widows. This would reason that Natasha was not given her trademark gauntlets in the MCU for graduating the Red Room program but instead developed them later as a personalized weapon. Yelena's could be considered "hand-me-downs" or based on those of her predecessor, which seems likely. Otherwise there is no reason for her to have similar gauntlets just because she also has the title of Black Widow. IIRC Yelena was also an understudy of sorts to Natasha so this would make sense if that were the case.
Reason #4: Cap's blood will probably be used to develop the serum Natasha uses to stunt her aging and remain at the pique of her physical capabilities more than half a century later. This could also be used to eventually kill off the ScarJo Yelena (who also dies in the comics), prompting the return of Bridget Regan as Natasha in the modern era to take back both her name and mantle. This is an easy way to swap out actresses without pulling a "Terrance Howard to Don Cheadle" type of recasting, and would be consistent with the lore. It would also be a logical thing for producers to do, considering that ScarJo's "Natasha" garnered criticism for her frivolity in Cap 2 being too out of character for a seasoned super-spy and former assassin. Considering a major theme of this show is female empowerment in the face of sexism, methinks it's what producers may be sewing the seeds for.
Reason #5: The training of Yelena to become the new Black Widow and a stand-in for Natasha could be the story of a Black Widow movie, as well as the circumstances of their rivalry, especially considering that the Ant-Man movie has a similar premise, starring the second Ant-Man while referencing the original. http://img4.wikia.nocookie.net/__cb20070407162912/marveldatabase/images/0/08/Black_Widow_Vol_1_3_Textless.jpg Alternatively, this may be covered in a Winter Soldier movie.
Reason #6: Probably my #1 reason, actually, aside from "ScarJo acts out of character for a seasoned professional killer".
THIS IS YELENA BELOVA FROM THE COMICS: https://31.media.tumblr.com/464c0c9cafa31becbb8382116bbe8540/tumblr_inline_ndpwzseF2S1rzkse5.gif http://data2.whicdn.com/images/63460750/original.jpg
THIS IS SCARLET JOHANSSEN AS A BLONDE FROM "LUCY": https://s-media-cache-ak0.pinimg.com/736x/72/77/00/7277003691588d56a955276ad9551eab.jpg
THIS IS NATASHA FROM THE COMICS: https://dailypop.files.wordpress.com/2009/07/black-widow-jg-jones.jpg
THIS IS BRIDGET REGAN AS A REDHEAD: http://i.imgur.com/RjoSGSl.png
Tell me ScarJo doesn't look like Yelena, or that Regan doesn't resemble Natasha. Try it.
Now, aside from the more obvious likenesses, I'd like to point out that Yelena's gear (pouches, straps and that belt) more closely resemble ScarJo's outfits in the MCU, compared to Natasha's, which is more of a sleek catsuit and the gold discs on the belt also function as shurikens. Yelena has worn a variety of outfits but usually they were more tactical/front-line soldier-esque. She has also used dark-colored wrist gauntlets, as opposed to Natasha's brighter golden ones, which would be similar in tone to the Mink's automatic revolver. Methinks we will see a more iconic Widow out of Regan and that will be the REAL Natasha.