"Cave to internet pressure"
It's more like "understand the movie was going to absolutely bomb unless they make the design more appealing to general audiences".
People way underestimate how badly this design went over. I would hear people audibly groan whenever the trailer came on in movie theaters. I showed the Sonic design to my mom (who I can definitely say does not care in the slightest) and her reaction was just "hm. He looks kind of stupid, doesn't he?"
I don't see what the benefit is in pretending this was just a few dweebs on the Internet (like me) complaining about this, the reaction was universally negative. It's not like it was just the fact that it was different from the games (the new design, while closer, still has a few key differences), but the original movie design was atrociously bad. This movie would have tanked hard had it come out in the state it was in.
Now, it could very well still bomb. I don't subscribe to the "they made it bad on purpose so they could get brownie points for 'fixing' it later" theory, but yeah, that's probably going to be the end result even if it wasn't intentional. The movie will still likely be mediocre at best, but the next time they launch a new trailer it won't matter how cheesy the jokes are, or how inappropriate the music is or whatever, all the buzz is going to be about how good Sonic looks. They'll get butts in the seats just for that.
I get the people who are concerned about "oh they're caving into backlash," but come on. This is a super cosmetic change to something that was reviled by like 99% of this movie's intended audience, and let's not pretend like movies being changed in post in reaction to test screenings, etc. is anything new.