Change active theme on all sub sites to new theme?

I have all my sub sites using the same theme but want to change them all to something new.

I tried:
define( 'WP_DEFAULT_THEME', 'your-theme-name' );
to see if that would change the active themes on all my sites but did not. Is that only supposed to set the default theme for all newly created sites and NOT existing sites?

I have a new theme network enabled, is there a way to activate that on ALL my sub sites at once and not manually one by one?

thanks!!!