iCloud keychain problems - asking passwords, can't change password, can't unsync
Sorry to bump an old thread, but I found this simple solution. At least in my experience, the password for the default keychain cannot be changed, so:

1) Create a new keychain (File→New Keychain… ⌥⌘N). I named it temp and gave it a simple password.

2) Right-click the temp keychain and select Make Keychain “temp” Default.

3) Select the original keychain.

4) Edit→Change Password for Keychain [name]… should now be accessible. Select it and change the password to your heart’s content.

5) Right-click the original keychain and select Make Keychain [name] Default.

6) Right-click the temp keychain and select Delete Keychain “temp”