I tried 'Live Migration' switch From Ubuntu LTS to Debian 7 back when Debian 7 first came out. (I backed up my partitions first) The switch didn't go so well. The furthest I got after like a few attempts (by the way this was just for fun) was for it to boot into (mostly) Debian and give me a tty. Even though this switch failed, it might be possible for those that know a lot about the Linux packages on both distros, and a switch was made to a distro with newer packages. A lot of websites say you can't switch this way, and I don't think most people would want to do this, and it doesn't sound like you are familiar enough. (I think most aren't)
[back up partitions first!!!] It's a lot easier to set up a partition for 'home', and then format the old 'non-home' Linux partition(s) and start installing the new distro in this space.
Debian has a 'stable' version(currently called 'Wheezy/7.6' as of this post), and Ubuntu has 'LTS' version(currently called 'Trusty Tahr/14.04.1' as of this post), which is good for like 2 years. Perhaps you should try one of those. Instead of Debian 'sid' (which is also called "unstable"), or Ubuntu's 6 month non-LTS version.
[ops... didn't realize this thread was old when I posted this]