It is simple to preserve /home when upgrading; just be sure to select an installation option that allows you to pick and choose which partitions to format, and don't (re)format /home. I do this everytime I upgrade.
As for converting partitions from ext3 to ext4, the process is described in the
Ext4 Howto. I haven't tried it yet, but it's on my list of things to do.
Daniel