Have Only One Partition And Want to Change OS! [duplicate]
You never want to lose your personal data, and you are never going to lose it: you should have one, preferably two backups.
Even if you could install the new OS without installing, you should have a backup first. Something may go wrong while changing your system.
Yes, to change the operating system installed on a partition, it is preferred to reformat the partition. So you will erase your personal data as well, and after installation, place it back using one of your backup copies.