Until and unless you format the drive ,no personal file is going anywhere irrespective of what version of Windows you are installing .
Now if you upgrade you will have personal files plus installed program
And if you clean install you will only have your personal files
OP is downgrading, C:/ will need to be formatted.
By the looks of it, you will need to use EaseUS Partition Master Free. Appropriately resize C: drive for the OS + Programs, so that enough space for new updates and software.
The new volume will store all your personal data, you can use the Library view structure in File Explorer to relocate folders; Documents, Pictures, Videos, Music and Downloads. Right-click Properties and Location.
You can follow the guide I posted above. Remember to Apply changes. Do not use your computer during this process.