I've done the same thing with in-place Fedora upgrades over a similar time-frame. It is impressive, but not unrivaled in the Linux world. (I'm not playing favorites with Linux here: I'm a FreeBSD developer.)
There's also a famous video on youtube where some guy installs every versions of windows (maybe not 1 and 2) back to back and it kinda works (passing custom configuration most of the time too)
mergemaster is still a drag
FreeBSD 10.0 and newer include etcupdate (and it has been available as port even longer) which improves a lot upon mergemaster through the use of 3way instead of 2way merges. Recent releases also added ways to keep most of your config in your own files (often included from /usr/local/etc) to avoid merge conflicts. Simple systems can often upgrade without any merge conflicts at all.
etcupdate is great!
Wish it was the documented way to do that step in the manual.
I'm also using etcupdate. It's also the default for beinstall(8), a very nice tool to install updates into a separate boot environment.