Luckily, I have a backup of the old .ipa on iTunes (which was v7.21.1). I understand Instagram's change to the new icon to try to fit in with Jony Ives flat UI but, it's too radical. I'm really getting tired of this flat UI. On a surprising note, I found out that the new update supports iOS 7 and greater. Pretty Interesting since the last iOS 6 compatible update (v6.5.1) does not support the newer Instagram Direct Message system.
So far, I hear more complaints than compliments about the update. Maybe Instagram will revert back to a similar one?
On the bright side, you can create a shortcut to the new Instagram app without jailbreaking. I've done this to a few of my apps.