Whenever I install a new distro, I never format /home partition. Also, I rename my home directory to <username>_old and create a new one. I start with mv of the most common files/directories like .bashrc/.mozilla etc and gradually move more as I need. This way after 2-3 months, I've left the old/unused cruft behind and can remove the _old home directory (in practice I keep them around because the size is small since I keep most of my documents in a separate partition and just symlink to that). If I'm changing computers, I restore the home from daily backup, rename it to _old and do the same thing.