Linux Mint Forums

If your home directory is on it's own partition you should be able to format / without loosing anything. If you are running any servers you might want to grab the configs for those /etc/samba/smb.conf /ect/ssh/sshd_config are two that I always backup, but only because I specifically set those up. If you aren't running any servers/don't know what I'm talking about then it's all good.
When switching versions of Mint keeping your home directory will keep most of your program settings, but since KDE tends to use it's own KDE-specific apps most of your stuff will be unconfigured when you install.

After you switch, if you have any KDE apps, you could install them into Mint/Gnome to collect your data (if a Gnome app can't use it )
--then switch to a Gnome equivalent

I could copy all hidden folders in /home onto my external harddrive, and only retrieve the stuff I need. The rest can be delete. This way the system don't get overloaded with unused crap.

That's actually a really good idea and I was going to suggest it, but didn't know your skill level. If you were really new, sometimes suggesting stuff like this can create more trouble than it's worth.

Some command line stuff that might make things easier: To move back: