I disagree on how the gameplay trailer says, "guess who's back" as if Sonic from the Genesis days is different from Sonic today. They're the same character for crying out loud. Did Disney do this with Epic Mickey when they bought back his original 1920s design? No.
It doesn't make sense how these two Sonics look different. They should look the same. Just face it you nitpicking spoiled brats...Sonic's appearance IS RETCONNED. Are they gonna have to explain why Sonic and his friends were 5 feet tall in Sonic The Hedgehog (2006)? No. Are they gonna have to explain why Knuckles had a pinkish color to him with different color shoes on as an antagonist from Sonic and Tails' perspective in Sonic 3 & Knuckles, but slightly changed his appearance when you played as him? No. Are they gonna have to explain why the Master Emerald change sizes from Sonic Adventure to Sonic Adventure 2? No. Are they gonna have to explain why Egg Golem from Sonic & Knuckles looks so much smaller than he did in Sonic Adventure 2? No. Are they gonna have to explain why items aren't placed in a TV box anymore? No. The simple explanation is that Sonic and other characters, and other items are REDESIGNED. It happened to Mario, Megaman, Mickey Mouse, Street Fighter characters, Mortal Kombat characters, Kingdom Hearts characters, and I could go on and on.
Amy had a flashback in Sonic Adventure when she was saved by Sonic from Metal Sonic. A scene that was directly from Sonic CD. Yes, a "classic era" Sonic scene. Neither Sonic nor Amy had their "classic" looks in that scene. Because of this, it doesn't make sense for Sonic to have two distinguished appearances if he's gonna meet his counter self from a different timeline.
And for those who are gonna say, "This is the Mobius Sonic". Don't make me laugh. Cannon Sonic was always and will be on Earth. According to the manual for Sonic the Hedgehog ('91), Green Hill Zone is referred to as "South Island". If you look at the map (or stage select screen) in Sonic Adventure 2, then you will realize that Green Hill Zone is an island located south of the map, below to the left of Green Forest. Sonic was on Earth since the genesis.
It's quite pathetic how a simple design can convince people this will "bring Sonic back". It's the gameplay that counts, not a design of a character.