Oh boy.

So you used some automated "click here to upgrade" button to do the upgrade? Yikes. That usually wipes out customizations, and takes no account of other things that also need updating.

And yes, you can tell your store to use another installed template by going to Admin->Tools->Template Selection. In your case, if Apple Zen hasn't been tampered with too badly, you might be able to get it to display. Granted, if it was the active template in the old store, it should have already been selected for you. Or, it would have been if you were using the same set of tables (see my previous post on prefixes) as the product/category/etc data were in.