I can understand why they didn't support upgrading from 32 to 64. Even in Win7 if you want to change from 32 to 64 you have to do a custom clean install and manually move your data and reinstall. 

The permutations of stuff they would have to support goes up quite a bit. For example the 32 bit registry is different from the 64 bit registry so migrating from 32 bit to 64 bit would be much more complicated. 
I think what the current upgrade install does is move all your existing data to a safe location - I read that it uses hard links to modify the location rather than copying files Then it install the new version of windows, and finally moves the user data back to the final location. This would be a lot harder to do when going from 32 to 64 or vice versa. 

If you want to change bitness you will have to do a custom install of Win8. Not sure if the image you get when you upgrade contains both 32 and 64 bit versions though, so before buying you may want to call Microsoft and inquire if this is possible at all. 

Edit: Apparently not possible. See this link

If you want the upgrade price you will have to update Win7 to 64 bit first and then do the Win8 upgrade. 

Best is to add new disk to system for w 8.

Why?  Cuz deleting the old partition, someday, can be more than annoying. (unless you use a good imager)

2nd best: new partition

Before you do anything: update windows, disk clean, create disk image.

And::::  w 8 does NOT contain a disk imager, only file backup.  (I have been using macrium for years.)

I currently have a 4 boot system for the audio computer. For testing and learning: w7 32 and 64, w8 32 and 64.

During the past 2 months I have performed in-place and new installs, all have worked fine.

At this moment I am upgrading 2 client computers: one a w7 in-place, and the 2nd an XP new install.

The install process identifies programs that will not work and demands uninstallation.

Download the w 8 upgrade advisor to get a list of potential issues. Get the new versions to be ready.

But, I recommend a new installation.

install win 8 clean - it just works better that way

install win 8 on an SSD - thank me later :)

the current price for early adopters is a bargain, grab it whilst you can. You also get a free Windows Media Centre included in the price - you need this as it installs some codecs. 

Don't bother waiting for SP1 - win8 is far more stable than win 7 ever was 

don't carry your Win7 apps/programs over - if you've bought an SSD you can install programs in seconds not minutes - it's worth the time to guarantee a stable system

Check your sound/audio card's compatibility, particularly if you are using a Creative card - their support is woeful

don't expect your disk utility software to work properly or anything like magic drive which mounts iso files - a lot of that stuff is now built into win8

DO NOT use Norton Ghost to back up your drives in Win8 - it will install but not let you register (gain access to it's features) and if you attempt to uninstall it will corrupt your system and possible delete NTDLR leaving you to reinstall

Well, I am a masochist. I couldn't sleep at night, I just have to keep up with the field (I wrote my first program in 1968).

So I went to Best Buy and asked if they would match Amazon's price and they did ($49.99 for the Win 8 Pro DVD).

The DVD includes both 64 and 32-bit versions.

So the day before Christmas I did my backup and on Christmas morning I started the install at about 7:30 AM.

It was not very easy.

I had to uninstall Acronis True Image 2011. When I did that and restarted I got a blue screen, so I had to restore the backup.

I downloaded the Acronis Cleanup Utility and spent the morning hunting through the registry for Acronis hooks. It puts file system filters in devices that don't get uninstalled (and they are set to run on BOOT, so you have to remove everything before you delete the files.

The cleanup utility made Acronis Disk Director useless, so I uninstalled that as well.

For a weird reason which I discovered later Win 8 required an uninstall of Perfect Disk 12.5 which is supposed to be Win 8 compatible.

This lead me down a nasty path of uninstalls, reboots, and tracking down problems.

Eventually I got to the point where Windows 8 install didn't complain any more so I made a special backup of the system drive in case I needed to revert back to that point. Then I started the real installation.

So I spent the Christmas morning from 7:30AM until 1:00PM getting things prepared.

When I started the installation it was 12:50 AM and it completed at 1:45 PM - taking about an hour.

I started personalization and set up, there were a few gotchas.

I had to re-install LoopBE30 - the MIDI port program that lets applications pipe midi messages to each other.

My Task Scheduler jobs were not transported, so there were several things that don't get loaded after I log in - I will get to them later (things like KeePass 2, for example).

I had to install the newest Focusrite MixControl 3.1.

I had problems with it unless I set Firewire delay to Medium (under Short it crackles). Latency is at 256 samples for 48kHz.

I had to setup the sound stuff, then ran Windows update and got a bunch of updates. Googleupdate decided it was a good time to automatically update Chrome while this was going on, so that also led to some weirdness.

I restarted a few times and finally got Sonar X2 and Komplete software synths running.

Latency (dpclat and LatencyMon) looks pretty bad compared to Win7, but I'm sure I can fix that part with a little experience.

The PerfectDisk fiasco got solved last and with a bit of difficulty. It turned out to be a weird issue:

When you install Perfect Disk, it unpacks itself in a directory in Program Files unless you specify otherwise.
I had originally installed Perfect Disk 12 for Win 7 and later got an update patch to 12.5.

When Win8 looked for incompatabilities, it looks throughout the entire Program Files folders, not just installations, so it was complaining about this older install file left around. That version is incompatible and Win 8 won't let it install, so I could not apply the patch to 12.5.

I had to download a trial copy of Perfect Disk and enter my license.

So it is now about 12 hours later and it's about time I did my first backup of Windows 8.

I have decided not to install any Acronis products - they are far too invasive despite their utility.

I have made Bootable DVDs and I can run the Linux standalone versions of Acronis True Image 2013 and Disk Director 11 from that. That has several advantages - but it is a bit less convenient.

Acronis left the Windows Backup setting in a weird state so I have to reset that - that will be another day.

I imagine I will be tweaking this for some time...

Lucky for me I did a backup (or two) first.

And now for another backup.... Merry Christmas!!

Dpclat is not accurate under Windows 8. Win 8 has low system latency. 

go with the CLONE FIRST idea. this is really the best way to do it IMHO.

I too bought into the MS "upgrade without issues" to win8--- against my better judgement--- and jumped at it. the startling thing was that for the most part it worked! everything "just worked" like you would think, with two major speedbumps:

-maschine hardware would not install no way no how
-roland fantom xr drivers would not install either.

so I did a clean format, and this helped maschine usb drivers to load automatically.
still no fantom drivers, so I googled it, and found a hack for it via these forums and its worked flawlessly.
im not 100% certain (no scientific tests yet) but everything runs smoother and faster.