Changing password of admin account from inside the root (superadmin) account
The password for an user account is stored separately from the keychain password. Usually both are the same password.

When you normally change your user password, you have to enter the old password and the new password. The software then uses these passwords to attempt to change the password for the keychain. This only succeeds when the old password for the user is the same as for the keychain.

If you reset the user password without entering the old password, then the keychain password will remain the same. 

So, if you do not know the keychain password, the only way to gain access to the keychain is to reset the keychain password. Unfortunately, this will erase the keychain.