Simple theme switch with parameters

I’m trying to switch my theme to support a cobranded advertiser. I’ve created a plugin that seems to accept my parameter but I can’t get the theme to switch.

Without boring you with the entire code, the important bits are:
(if my parameter is detected or the cookie is set then…)

add_filter('template','change_template');

function change_template($template = '') {
	$template = '123cobrand';
	return $template;
}

What am I missing?