So here’s what you’ll do now. If you edit a picture (by applying a filter, for example, or adjusting the color), you’ll hold down the M key while you’re still in edit mode to see the original; release that key, and you can view your edited version for comparison’s sake. So here’s a picture I added a filter to…
…and if I hold down M, I’ll see my original image.
Hmmm, I think I like this one better than the “emo pelican” version.
One of the ways this feature is different from iPhoto’s Shift shortcut is that you can’t compare any cropping you’ve done. Crop an image and press M, and the program will stubbornly refuse to show you the original (and you’ll hear a “you can’t do that” alert sound for your trouble). I really hope that Apple puts that particular ability into Photos—I used it all of the time with iPhoto. It is nice, though, that we’ve still got the former Shift trick, even if Apple decided that they wanted to obfuscate how you do it!