Yes. You can.
Apple says, "The Retina display’s pixel density is so high, your eye is unable to distinguish individual pixels."
That's a lie.
Obviously, youre going to see the pixels if you look closely enough. The whole concept of the Retina Display (which is just a marketing term) is that your eyes cannot make out individual pixels at a certain distance from the screen. Apple clearly states that the pixels are undiscernable if you hold the phone twelve inches from your eyes.
Back on topic: I don't need to upgrade my iPad 2, but I do a lot of reading and the retina display would be nice. Could I live with the 2? Yep.