While Ubuntu/Mint is based on Debian .. . they are not interchangeable nor compatible.
Ubuntu has made too many changes to the Debian base in their quest to make the distro more 'user-friendly. As well as, adding and changing config files, ppa's and repros that are not compatible with Debians base or philosophy.
Only the core Linux Kernel is the same in both distros.
If you're using the same desktop in both distros (ie, Gnome), then many of your home folder files will be compatible, especially data files and settings such as for Evolution, OpenOffice, browsers, etc. Copy your home folder (including hidden files and sub-folders) to a thumb drive and do a fresh install. It's worth it.