You can use the set_user_role
hook, that will only fire when the user role changes:
add_action( 'set_user_role', function( $user_id, $role, $old_roles )
{
// Your code ...
}, 10, 3 );
If you want to restrict this to a profile update, you can use:
add_action( 'set_user_role', function( $user_id )
{
add_action( 'profile_update', function( $user_id )
{
// Your code here ...
} );
} );