A question that I have had for a while now.

I believe the games in your account aren't affected.

nothing happens to the games you already own but you will need to change your store region settings if steam doesn't change automatically.

I did this when I moved to China. No changes to already owned games, even the ones that aren't available in this region. My wishlist contains titles that aren't available on the Steam Store--they still appear on the wishlist, but just won't load up. Overall, I'd recommend to go ahead and buy anything(s) from your current region that you may want if they aren't available in the destination region, since I wish I'd done this with some of the Final Fantasy series that isn't sold here.

Now, what would happen to any games you have had since before the move that are not supported or allowed in the new region?

If you bought or activated a game in the last 3 months, and the license has run lock on the destination country, the game will be blocked until you have owned the game for more than 3 months. Otherwise, the game runs normally, with the content of the original license (for example, games which has a censored version in Garmany when bought in Germany will stay censored wherever you are).

If you move, do you have to change your system's region settings?

No. If you move to a more expensive region, or a country with more restriction on content, you can opt to keep your original country's region. Of course, you need to have a payment method/billing address in the original country to buy anything from Steam store.

Would you lose access to said games that aren't supported or allowed in the other region?

Like said above, you won't lose access to the game. However, you will lose access to the store page of the game and achievement listing (among other things) in a restricted country.

One thing you didn't ask but I'll address it here anyway. If you have region restricted Steam key, key activation is tied to IP address. So if you live in Germany with US store, you won't be able to activate the key with activation lock in Germany. You can still buy the game from Steam store, and it will be the ROW version.

Can only comment second hand, but one scotsman i know who moved to germany still has his steam country (from STORE & PURCHASE HISTORY page) set to UK and can buy all the good stuff. No matter the german IP, i think he´d have to fuck this one up for himself by fiddling with the "Update store country" button.