Is it possible to once shot upgrade to the latest version, with minimal issues?
Yes... but... a simple backup is always a good thing, just in the case something goes wrong...
Another "thing": even if the upgrade should be fine, if you don't have to much stuffs installed, a fresh install on another card (or better to a new one, the flash cells in the card have a limited life, maybe the old could be at the end of life) always is better...
And a final "thing": the LE12 have no more updates. So in the case of fresh install, you should try the latest LE13 nightly, it's good for the daily use and have newer software/drivers.