Yes you can, BUT that would also remove all the adjustments you made after cropping, so you'd have to make detailed notes about what they were and then re-do them.I think each adjustment is recorded by itself so you can just back up to the adjustment right before your crop if you want.
That's the right approach. Cropping in Lightroom is non-destructive. You're not throwing away the pixels outside the crop area, as you would be if you cropped in Photoshop; you're just telling Lightroom not to show them to you for now. But they're still there and if you re-adjust or even remove the crop it's like the first crop never happened.Or like he said just readjust the crop by opening the crop tool again.