I have something like this, it removes all capabilities before adding a new role. You can modify this to achieve what you want when changing roles on frontend.

add_action('frm_after_update_entry', 'update_user_role', 10, 2);
function update_user_role($entry_id, $form_id){
    if ( $form_id == 2 ) {
        
        $userid = $_POST['item_meta'][19];// ID of the userID field
        $role = $_POST['item_meta'][236];// ID of the role dropdown
        $accesslevel = $_POST['item_meta'][270];// ID of the role dropdown
            
        if ( $userid && $role ) {
            $user = get_userdata( $userid );
            if ( $user && ! $user->has_cap('administrator') ) {
             	$user->remove_all_caps();
                $user->set_role( $role );
              	$user->add_cap( 'access_level' , $accesslevel); 
            }
        }
    }
}