Does "Change..." option in "Something Else" during installation erase the partition contents?

Now, I am unsure if selecting the root partition with all of my data and changing it to / mount point would delete or keep all of the data. How should I proceed further?

The / mount point is formatted if...

• the filesystem is not POSIX compliant (ext4)
• you did anything with the boundaries of the partition
• you click the "format" option.

To proceed: mark the system partition as /, don't pick format.

And of course before you proceed with the above you have made sure to have a backup of your personal files so if anything goes wrong you can still restore your files from that backup.