And if you make it private again, do you know if it will go back to the earlier state or if you have to continue from the realms current state?
sharing → making private → sharing again → 
making private again
Is it this Backup Feature or did you mean the old Backup Feature for Realms before Co-Op got released?
Be sure to read through the instructions before you download OR attempt anything so you know if it would help you or not!
https://forum.norestforthewicked.com/t/patch-notes-together-hotfix-9/26273
You can always check if you can download your old save from your own Steam Cloud Storage?
It can be a bit tricky to find the Steam Cloud Storage so i usually access it through a Browser instead of the desktop app