How will they explain that Alura will have different look since eric durance is takingover the role in season 3

They don't have to reshoot any scenes with Alura for the beginning, because they no longer use her face in the voice over by Kara...they haven't since Ep. 1 of the S2. The last time we have actually scene Alura is Ep. 4....Also, using the young actress who played young Kara would be weird considering those scenes with her were filmed all the back in March, 2015....if anyone knows anything about teenagers, 2 + years is a huge difference in height, look, everything...so why waste the money reshooting the beginning scene when there really is no need to.

There really is no need to do anything except bring on Erica...

I agree, also they sometimes fix that sort of thing in syndication. For example on Seinfeld Jerry Stiller plays George's father, Frank Costanza, but in an early episode (before Frank Costanza became a recurring character) they had another actor in the role. So when Seinfeld was syndicated, they simply reshot those scenes with Jerry Stiller.

I have no idea as to how syndication works or whether the CW has plans for syndication, but still there is no reason they couldn't do the same thing for future DVD sets of Supergirl seasons 1 and 2, as a special feature.
Alura doesn't have that many scenes so it shouldn't be that difficult to reshoot them with Erica Durrence.

If you were to reshoot all of Alura's scenes with Erica then you would have to also do Astra's scenes too because they were identical twins. I suppose they could change it to fraternal twins but that has its own issues with Kryptonian lore. Another big issue is the actress who played young Kara is working on another project and would be unavailable for reshoots. The show will only be able to use her for Kara on Krypton scenes for maybe another year before she's too old. Change the introduction to take Alura out and move on, don't spend a cent reshooting.

I doubt it'll be brought up. They'll likely just go on as if the recasting didn't even happen. I think the recast will be accepted by the GA without much fuss, it's us, the fans of the show, that'll take some time to readjust (or not).

Us fans are just going to have to get over it, the faster you do the better for your enjoyment of the show. There is a perfectly good real life explanation with no hard feelings from anyone involved. It should help that the actress look enough alike that they could easily play sisters.

I agree, also they sometimes fix that sort of thing in syndication. For example on Seinfeld Jerry Stiller plays George's father, Frank Costanza, but in an early episode (before Frank Costanza became a recurring character) they had another actor in the role. So when Seinfeld was syndicated, they simply reshot those scenes with Jerry Stiller.

I have no idea as to how syndication works or whether the CW has plans for syndication, but still there is no reason they couldn't do the same thing for future DVD sets of Supergirl seasons 1 and 2, as a special feature.
Alura doesn't have that many scenes so it shouldn't be that difficult to reshoot them with Erica Durrence.

Actors/actresses take over U.S. roles all the time in daytime television with no big deal. On Young and the Restless, 4 people have played the same character in just 8 years. lol

I doubt it'll be brought up. They'll likely just go on as if the recasting didn't even happen. I think the recast will be accepted by the GA without much fuss, it's us, the fans of the show, that'll take some time to readjust (or not).

Us fans are just going to have to get over it, the faster you do the better for your enjoyment of the show. There is a perfectly good real life explanation with no hard feelings from anyone involved. It should help that the actress look enough alike that they could easily play sisters.

No explanation is needed and none should be given

Sometimes no explanation is better

I doubt it'll be brought up. They'll likely just go on as if the recasting didn't even happen. I think the recast will be accepted by the GA without much fuss, it's us, the fans of the show, that'll take some time to readjust (or not).

Piggybacking off of Brierrose's comment, another example of a recast where everyone just went with it is The Fresh Prince of Bel Air. They recasted the role of Aunt Viv due to conflict between the actress and Will Smith. Everything continued on as normal after the change in actress. So I'm sure it'll happen here with Supergirl too.

They'll probably just go the Darren on Bewitched or Becky on Roseanne rout. On both those shows actors were replaced and things just went along like nothing happened. On Roseanne, the reason was the original actress wanted to go to college full time. I seem to remember the original actress came back then left again. They did do an after the credits gag once with the two Beckys as an acknowledgment.