To add to William's great points, if you're only optimizing for the country because you want to sell the shirts there, you'll have to do more to rank. I'd recommend blogging about football in those countries, setting up country-specific landing pages with the shirts from those countries, and add pages about shipping to those countries and how much it usually costs. I hope that helps!

Have you read SEOmoz's excellent guide to redirects? If not, look that over.

In my experience, it doesn't destroy traffic to move from a .somethingelse to a .somethingnew. I've moved from .ca to .com, vice versa, .net to .com etc.

Things to consider:

• Have a crawl of the site (crawl test is great for this, plus I use Screaming Frog to cross-reference)
• Double-check that all the URLs are redirected and working (you can use a test server for this)
• Check analytics (use the last year of data) and look at all the URLs that received even one visit, and make sure they're reflected in the data from the previous two points.
• Sign up both URLs in Webmaster Tools and indicate the change there when it happens. I'd recommend parking the new .com domain on the old .net domain for a bit, registering it in Webmaster Tools first, and then having the switch happen.

Let me know if that helps.