After two days of slow downloading, I finally have my Ubuntu DVD and I'm ready to make the switch. My harddrive has been split into 4 partitions for Linux: boot, root, home, and swap.

Does the Ubuntu installer have an option for replacing the OS but keeping user accounts from another distro? I have about 10gigs of files in my Mandrake user account alone. My plan is to replace the Mandrake OS with the Ubuntu OS, copy some of my files and settings from my old account into my new one, and manually delete my old accounts.

Does this sound like a good plan? Your advice would be greatly appreciated.

If the size of the present home partition is adequate, you could simply not format the home partition. If you want to simply copy files that you want to the new installation, you could first rename the user's home directories. For example appending "-old" to the names, and reuse the same home partition. Then after the installation you could move what you want into your new home directory. You may need to su to root and use the chown command. Different distro's may have different UID ranges. For example, Mandrake starts at 500 while SuSE starts at 1000. There are two places where you will find your system settings. A) System wide settings will be contained in "/etc/". You might want to make a tarball of the /etc directory. Using a program like ark or mc, you could read an old configuration file without even needing to extract it manually.
Other settings would be contained in hidden directories on your home page. For example: .kde, .pam and .ssh. If you leave the old home directories present, you could simply refer to them at your leisure.

If I remember correctly, Ubuntu is slackware based. So many things in the /etc directory will be quite different. For example, different startup scripts are run. Slackware doesn't use the chkconfig command. Also Mandrake uses PAM and the shadow suite to control program access.

Thanks a lot for your reply.

I think my main concern is can I install Ubuntu to my 10gig / partition, and set "Home" for my 70gig partition without wiping everything that is currently on that 70gig partition. Is this possible?

Just make sure that you elect not to format the /home partition. That usually means that you want to use an "expert" partitioning option to have manual control. So don't just choose a drive and have it autoconfigure the paritioning. I haven't installed Ubuntu but I'm sure you can decide what not to format.
One potential problem is if the old home parition uses a filesystem that the installer doesn't understand.

The ubuntu-installer understands ext2/3, xfs , jfs, reiserfs.

Done. I'm using Ubuntu now and my files weren't touched. Thanks a lot for your help.

no... it's actually based on debian