Can I make an upgrade from a german to an english version? Can I use the german Serial No. for upgrade to an english one?
Yes you can do that.
Please refer : http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platfo rm-language-swap.html
You can contact Adobe Sales at the number specified at the bottom of the page : http://success.adobe.com/en/na/programs/rfi/dm_sales_inquiries.html
thank you for that info!