I have new blog templates plugin installed on my multisite

I have new blog templates plugin installed on my multisite and it works well. However my issue is that when I think of a devastatingly brilliant addition and add this to the default template it only activates on blogs created after that point, while existing blogs are denied my latest whizz-bang idea !

Would hard coding the default theme with the new stuff apply the changes to existing blogs? Is there another solution?

Thanks