Something I've asked about in the past on this list, and at MUUG meetings, as some may remember: How to upgrade an entire system from 32-bit to 64-bit x86_64 in-place, over a network, off-site. Specifically for Fedora (and RHEL). And ensuring the resulting system was "clean" without a mess everywhere.
Well, I finally developed a method and have successfully upgraded all the boxes I'm responsible for. And for not a single one was I required to go on-site! Yay!
I wasn't sure it was possible, as all Fedora forums said it was hopeless, and any instructions out there were for like Fedora 5 or 10 and relied on features and workarounds that are no longer present. Most people are like "just reinstall from DVD". Great if you're on-site and your system isn't heavily tweaked. Even then, you're looking at blowing a day (at least) on the upgrade. I couldn't do that for 20+ off-site systems!
So I found new hidden methods and workarounds and tested and honed them to come up with a reliable procedure. And it worked! I now have boxes that have been successfully upgraded, using yum/dnf, from ~FC2 through to F24+ and multiple iterations of hardware from P1 to today's Xeon; never with any on-site upgrade or reinstall! And there's no mess & no leftovers! Try that on Windows!! (Boy, do I love Linux!)
I'm mentioning it here for two reasons: 1) curiosity, as I'm sure at least someone wants to hear about the outcome of this loony proposition; and 2) I recall at least 2 members telling me they had a similar need (see, it's not just me!).
I thought about doing a presentation or something but it would be so narrowly-focused that it would bore the 99% of have no need of this. So for now I'm keeping it under wraps and will be selling it as a service to those who have the need (gotta keep my computer company in business, after all, it is my day job!). So if you want info, email me privately; special uber-discount for MUUGers!
P.S. The only other distro I could find with easy 32->64 instructions was Arch. Other than that it appears hopeless using online resources, though I'm pretty sure you could do the upgrade easily on Gentoo also.