Change Schengen visa to long-term work visa in Germany?

You generally cannot convert a 90 day Schengen visa into a long term work visa while in Germany. A Schengen visa is meant for short stays such as tourism, business visits, or short training, and it does not allow you to take up long term employment.

If you want to work in Germany for more than 90 days, you normally have to return to your home country or your country of legal residence and apply for a national long stay work visa (D visa) at the German embassy or consulate there. This application requires a valid job offer, proof of qualifications, and, in some cases, recognition of your professional credentials in Germany.

There are very limited exceptions where a local immigration office (Auslanderbehorde) might allow an in country status change, but these are rare and usually apply only to highly skilled professionals, researchers, or people with special circumstances. Even in such cases, you would need strong justification and prior approval.

In most situations, the proper and accepted route is to leave Germany and apply for the work visa from outside the country.