Is there any way to upgrade from a German Vista Ultimate to English Windows 7 without a clean install?

"There will not be a different version of Windows 7 Ultimate in German.

The reason why I'm saying that is that Windows 7 Ultimate (as well as Windows Vista Ultimate) comes with the ability to switch the language after the installation using language packs. All you have to do is install the English distribution and then install the language pack with the language of your choice (a German language back is available already for Windows 7). File wise, both the English version and the German version has the exact same file signatures.

That being said, you can upgrade to Windows 7 using the normal upgrade route whether your Windows Vista is in English or not since your Windows Vista Ultimate in German is just the English version with the language pack installed. Before upgrading, open the Change display language item in the Control Panel and switch your display language back to English. Then switch to the Administrative tab, and use the Copy Settings button to copy the language setting to the SYSTEM and DEFAULT accounts. Windows 7 will then proceed to upgrade without complaining.

You might need to install the language pack after, which is on a separate DVD marked as Windows 7 Language Pack (x86) - DVD (Multiple Languages) (or Windows 7 Language Pack (x64) - DVD (Multiple Languages) for 64-bit) in your MSDN Downloads.

Edit: After verification, MSDN also provides a DVD of Windows 7 Ultimate with the German language pack already installed, marked as Windows 7 Ultimate (x86) - DVD (German) (or Windows 7 Ultimate (x64) - DVD (German) for 64-bit). Then again, the only difference between the German and the English version is the fact that the German download has the language pack slipstreamed in the disk."