How to know which iOS apps are restricted to a country?
I don't know of anything automatic - but you can browse the iTunes store here:

http://itunes.apple.com/us/browse

This is the US store - if you browse to your app, and then replace the 'us' part of the URL with the country code for the store you want to check (e.g. 'fr' for france, 'ie' for Ireland, etc) - you'll either get brought to the entry for the App in that country, or get an error message that the app is not available.

I don't think you'll find a huge number of apps that aren't available.

If you use Apple TV, do consider that the films/TV choices in other stores may be much more limited.

You might want to consider keeping two accounts rather than changing. It can make management a little more inconvenient, but you'll have a bigger choice of things to buy/rent.