Germany how to change PhD visa before defense to a working permit visa?
Under certain circumstances, it is possible to change one visa type for another. However, immigration law is tricky, so the first thing you should do is check with your local Ausländeramt, who will be able to tell you what is allowed and will have a list of the necessary documentation to apply for a visa, but at a minimum you should expect to need everything you had to provide to apply for the student visa, plus a copy of the job offer. You may need to get a "certificate of good conduct" from your Bürgeramt, but I'm not certain about this.

One of the things to keep in mind is that there is sort of a catch-22 involved in the visa process: technically, the visa isn't valid until you get the job, and you can't get the job until you have the visa. So the job offer is the "promise" that the employer will hire you if the visa is granted.