Linux: in place upgrade to 64-bit x86_64 from 32-bit abi

So, I have a small project I finished. Took me about two weeks of off  
and on tinkering here and there on my work desktop. I spent probably  
about an hour a day, sometimes off hours (via remote vpn), sometimes  
no hours in a day, mainly on work days. This may not be useful to some  
of you that have IT departments with piles of computers. However, this  
is a cable company. Our IT department hates talking with us and uses  
red tape to prevent requests.

All in all, I have about 8 hours of work in this. I was able to  
perform this upgrade WHILE working, and performing other duties.  
Sometimes, I was remote. Sometimes, I was at my desk.

The only downtime I had was the three reboots (initial 64-bit kernel,  
boot into 64-bit install, final reboot to ensure settings).

I had one burp in that the new gentoo renames my ethernet interface to  
something like enp0s25. Not a big deal to update.

[unrelated note, but it slightly complicates the upgrade]
My system's LVM partition is LUKS encrypted using a keyfile (dd  
if=/dev/urandom) that I store on a small USB stick on my keychain. At  
reboot, I have to plug in my usb stick (backed up at home and in other  
places). If you don't have the keyfile plugged in, you can't access  
the main drive's partition at boot up.

* Gentoo Linux installed about 5 years ago, with semi-frequent updates  
over the years
* Intel(R) Core(TM)2 Duo CPU     E8400  @ 3.00GHz
* 8 gigs of RAM (using PAE extensions)
* 32-bit ABI (due to 64-bit support 5 years ago)
* 3.10.7-gentoo PAE SMP i686
* Luks encrypted partition
* LVM2 volumes including / on encrypted partition
* initramfs as initrd with rescue image and keyfile bootstrap

* Gentoo Linux updated
* 64-bit x86