Fedora and LVM

There's a simple non-LVM approach I've used a lot, which is to create two root FS partitions - install your current distro on one, then do 'rsync -avH' from one to the other, boot into that one, and do the upgrade there.

Or simply use a good backup tool to make a backup first - a 2TB drive is not much over $100 these days, just use rsnapshot or your preferred backup tool. If you run rsnapshot daily, it will complete very quickly on a root FS that doesn't change very much, and you can then easily do file-restores and inter-day comparisons of config files (not quite as nice as etckeeper but pretty good).

Once btrfs is more mature, its snapshots look really nice, and much faster than LVM's. Of course, backups are a really good idea either way.