switching from 32-bit to 64-bit

[sethg-Dp9fwfP21SfQT0dZR+AlfA at public.gmane.org (Seth Gordon)]

I have a desktop machine at home that used to run 64-bit Ubuntu.  Then 
the hard drive went south, so I got a fresh drive, bought an Ubuntu 
CD-ROM from the Micro Center, did a fresh install from the CD-ROM, and 
then upgraded over the network to the then-latest version.

At some point I noticed that I was running the 32-bit version of the OS, 
not 64-bit.  Is there a relatively painless way to move everything over? 
  (I need to upgrade it from 9.10 to 10.04, in any event.)