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.

Absolutely. I don't know how much work you've done with local SEO, but I'd also consider doing things like citations, Google+ Local pages, etc for each of your store locations.

And also create pages for specific country themes - like all the football shirts from France (assuming that's one of your countries).

Not quite. I think you should have country specific sections, but as a part of the main site, not on separate subdomains or what have you. You need to create something interesting that's rank worthy, that also is an integrated part of your main website.

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!