How do I migrate from Ubuntu to Debian?

First thing first, move that home directory to a new partition. It's really not that difficult to do (I made the same mistake, took ~30 min to do, most of it copying). I'll give you a simple step-by-step, but more detailed walkthroughs are available.

1. Create a new ext3/ext4 (or other fs) partition on a drive (eg /dev/sda3) in gparted.
2. Mount the new drive:
   • sudo mount /dev/sda3 /media/disk
3. Copy the entire contents of your /home directory to the new drive. There is some debate whether to use rsync or cp for this, but cp worked fine for me (using the -a flag as nagul pointed out to maintain file permissions and ownership).
   • sudo cp -a /home /media/disk
4. Edit your /etc/fstab file and create a new entry for /dev/sda3 and set its mount point as /home.
5. Move your existing home directory (don't delete it yet in case something goes wrong).
   • sudo mv /home /home_old
6. Now you can either unmount and remount /dev/sda3 to /home or just reboot the computer.

After you have /home on its own parition, changing your distro is incredibly simple. Just download a live cd and/or an installation disk for the distribution you want and install it on the same partition hosting your current distro (or on a seperate partition for multi-boot purposes).

Then, after you have the distro installed, just follow steps 4-6 again (if the options for a custom /home partition weren't available in the installer). Now you have all your old files, settings, etc but with a brand new shiny distro.

An over-the-top upgrade will not work.

Before you do anything else, backup your data (though that should go without saying).

Then you could:

1. Make space for a new install by shrinking and moving partitions
2. Install Debian in the newly freed space
3. Once setup mount the old partition somewhere and make /home a synlink to /home on the old partition.
   (note: file/dir ownerships will be wrong, as users will not have the same UIDs nor groups the same GIDs, you'll need to adjust them or mess around to make the UID->user and GID->group relations the same on the new setup as the old)

or

1. Wipe the system
2. Install fresh
3. Restore /home from backup
   (you may still need to mess with file/dir ownership)