Since you are using date base permalinks (making posts look like they are in a directory structure using year/month/day, you can simply block Google from reading them using your robots.txt file.
To block all bots, use something like:
Whatever period you want to block.
If you have a year where you want to be more specific (such as blocking the first 3 months of 2011, you simply add
If you only want to block them from Google, replace 'User-Agent: *' with 'User-Agent: GoogleBot'.
Check robotstxt.org for more description of robots.txt files. Or just check with a Google search.
Your template directories contains standard files that does things like display post loops, side-bars, and other things. All depending on what the actual theme does and looks like. You would simply create the new parent theme linked to the old theme with one or more files replaced with copies from your original theme, but modified with the new checks. But!
Since you can block using robots.txt, you do not really need to change any theme. Robots.txt blocks even better than the noindex meta, since the robot block will prevent GoogleBot from even trying to load those old posts.
If you still want to change your theme, there is a lot of documentation for theme creation and modification on Wordpress.org.
After finishing the robots.txt changes, you should go to your Google webmaster account and issue a removal request for the now blocked paths. Otherwise it can a long, long time before Google forgets.
Even with that, it will likely take a while.