Linux

Is it possible to change to the 64-bit version of an OS from a 32-bit version?

Mostly no. There are a few articles out there from people who have hacked distributions a bit to do it, but to be honest a reinstall is usually easier and safer and more consistent.

I figured as much -- I've been preparing to do a clean upgrade.

Since I haven't set up the partitions for dual-boot, would it be easier to replace the existing partition with the new OS rather than repartition as needed to have both? Is there any reason I should prefer one course of action to the other?

Your applications will dictate that, as some support 32bit only, and some perform better on 64bit. However, you can run 32bit applications on 64bit, however there are library conflicts that can be possible. You can always run a virtual machine.

Wow, Ubuntu made that easy! I didn't have to do anything, it just gave me radio button options to install it over the old OS or in a second partition.

I'm primarily running math programs which benefit significantly (+80% to +200%) from the move to 64-bit. So far only one of the apps I tried to install wouldn't work (an FTP program), but there were plenty of substitutes so I just went for a different one.