Perhaps you want a lower-level-of-abstraction reply.
All distros nowadays recognize existing partitions. DO BACK EVERYTHING UP. But if you are sure you can tell where your user data is (i.e. in which partition /home is), you can define the mountpoint of that partition as /home again, tell the installer not to reformat it, and the new install will (should) leave your data alone.
After the install, you may have to give yourself ownership of all in /home/<your-userid> to yourself again using chwon.
(Question for the wiser: IS THIS NECESSARY or will one system recognize the older's ownership associations? - i.e. is it a hashed, install-specific, or not????)
Also be careful with which filesystem you specify. It should be the same as in your previou install, with some exception: eg you may mount an ext3 partition as ext2 (without journaling) and perhaps reactivate and re-do journaling later (but it's not in general a good idea).
I would not even try to mix and match two different distributions. One distro is enough at war with itself already.
BUT you may want to do a copy of all files in /etc, where most settings live. Afterwards, you may use them (with caution) as an inspiration to change settings in the new system.
Moreover, be careful with /var. The older-style mailsystems put mail files there. So, you may want to treat /var along the lines of /home. There again, you may have little or nothing of value there. FWIK, tribal mailers like mutt, pine etc use /var, but Evolution, Mozilla and other modern ones tend to put the stuff in (hidden) folders in /home/<your-userid>.
(Wiser boys & girls, do you concur?)
Finally, OS changes: not all OSs support all filesystems. There are Linux kernels or modules with support for the strangest filesystems, and commercial Unices may support ext2/3, reiserfs or whatever, but you need to check.
Another argument in favor of going clean slate.
Filippo / spamhog
Computer Victim (as in "fashion victim"

- Milan, North Poldavia -
40% WinME, 40% Linux (Debian, Libranet, Vector, Lycoris), 20% Win98, trace amounts of Win2k, xBSD, QNX