You may disable all direct root login by editing the variable CONSOLE in /etc/default/login file as below
then you should login as regular user and use "su -" to switch as root.
Depends on the version of Solaris as well.
For Solaris 11 root is a role and you cannot logon as a role.
So you can change the role's password.
For other versions root is probably just a normal user account and you can change it however.