My keychain passwords gone after upgraded to High Sierra
I found the solution: I managed to restore my Password Entries permanently. It again involves tinkering with the Terminal, steps are as follows:

open the Keychain Access App
create new Keychain
save new Keychain as default keychain; quit Keychain Access.
// Hint: As soon as a keychain is not the default keychain anymore, you can change its Password again via Keychain Access (no more grayed-out menu commands there)
Continue inside Terminal:

bash$ security set-keychain-password login_renamed_1.keychain-db  
// this is setting a new password for the old (corrupted) keychain-db.   
// Important: you must know the old password for being able to do so.  
// In the next step I am replacing the default keychain-db with 'login_renamed_1.keychain-db'  
Old Password:  
New Password:  
Retype New Password:  
bash$ sudo mv login.keychain-db login.keychain-db-out  
// renaming the current keychain-db to &quot;*-out&quot;  
bash$ mv login_renamed_1.keychain-db login.keychain-db  
// renaming the old keychain-db to the name of the default keychain  


Restart Keychain Access. The lost 'Secure Notes' are temporarily restored to the Default Keychain, but only until the next login.
Choose Default Keychain, in the Sidebar choose 'Secure Notes' (I hope this is correct, as my macOS runs with german language).
Select all Secure Notes of Standard Keychain and move them via drag and drop to 'Secure Notes' of the new Keychain you created in Step 2.
now to the tedious part: in the now opening Dialogue Window you have to type the Password of the Standard Keychain – For every single entry (which was 133 times in my case). Tip: you may copy the PW and paste it in the Dialogue field instead of re-typing it.
Quit Keychain Access, restart Computer
Upon the next login all Password Entries are restored – Eureka!

Now I can safely copy all my PW Entries to another app which is more useful to the task, e. g. 1Password,
which is very secure and has a GUI, but you'll have to pay $ 3.99 p. month. I will give the Unix Program 'pass' a try, which is command line and requires some knowledge about git and github, but is very configurable and free.
In the end I'd like to say: “Thaaaank you, Apple.” It costed me only my nerves and ~5 hours work to clean up after you.
But it was still worth it, I’m happy :)