Select Finder from the Dock.
Select Applications > Utilities.
Search for Terminal under Utilities.
Open Terminal.
Type whoami. Note the username that will be displayed - I’ll mark it here as username_displayed.
I’ll mark it here as new_keychain_name the new Keychain that needs to be set as default.
Type in Terminal the command: security default-keychain -s /Users/username_displayed/Library/Keychains/new_keychain_name.keychain-db
(replace both username_displayed and new_keychain_name with your identifiers)
Quit Keychain Access if already opened.
Open Keychain Access.
Open Keychain Access.
Right-click on the “login” Keychain under “Custom Keychains” (which is your old Keychain). Now you can change the password on the old keychain.
Make the old one default again using Terminal command: security default-keychain -s /Users/username_displayed/Library/Keychains/login.keychain-db
(Again, replace username_displayed with your identifier)
Quit Keychain Access.
Open Keychain Access.
You can now delete the temp keychain: Right-click on the “new_keychain_name” Keychain under “Custom Keychains” > Delete > Delete
You'll see the new_keychain_name listed under Default Keychains in the left-side panel.