Switching fron Network enabled plugins to Site Enabled


I hope some one can help, although not specifically a WPMUDEV question there are so many experts here.

OK the situation is that I have has a plugin set up as network enabled so all my sites have benefited from it.

Now I want it to be an option for new sites. But of course I want all existing sites to keep the plugin.

So basically, unless I can find an alternative, I think I have to network disable the plugin, and then go manually through all the existing sites admin panels and activate the plugin.

Obviously with quite a big multisite that is going to be painful to say the least.

Any ideas?