One of the greatest ironies of the power of Lightroom is you’re more or less forced to keep your original image unsullied by your adjustments. I’m always preaching the blessing of that…even to those who are working strictly in Photoshop. But in Lightroom, as long as you haven’t poked the Delete key AND click on Erase from Disk in the resultant dialog, there’s no way you’re going to destroy your original after trying too hard to retouch a portrait and realizing that you’ve really ended up with something that’s quite unsightly. The few times I have had to poke at the Reset button, it’s been a real life-saver.
By the way, although you can use Lightroom to readjust an image you’ve adjusted in Photoshop and then had Photoshop save to Lightroom, you probably won’t like the results because all that 12- or 14-bit information has likely been lost unless you’ve made sure that Lightroom’s preferences have been set to save to a 16-bit image. Even then, if you’ve converted from 16- to 8-bit to use some of the Photoshop commands that don’t run in 16-bits, you’ve already thrown out all that extra data.