Germany how to change PhD visa before defense to a working permit visa? I have at the moment a visa for my PhD valid until 30.06.2018, but my defense will be in the end of July or middle of August. This week I have received a job offer (full-time) and I would like to change my present visa to a working permit Visa in order to start working and also wait for my defense. Is it possible? Does somebody know which documents are necessary for that?

Thanks a lot for the help and attention. 

SOLVED: 

As I always expect, I like to see final answers to questions I look for in the Internet. At first I would suggest you to look the answers for my question, they helped me a lot. Finally I needed, the bachelor diplom translated to german, the form of the blue card filled, the working contract, a form written by the employer which describes what work will be done and a photo. 

I went to the KVR on a Tuesday and in this day I could not solve much because It was necessary a permission from the Bundesagentur für Arbeit (https://www.arbeitsagentur.de/privatpersonen). This permission was requested on Tuesday and I received an answer on Thursday evening. Friday I went back with these documents again and could finally receive the Bluecard.

An important tip: check if your institution is in the Anabin database (https://www.arbeitsagentur.de/privatpersonen). If it is not, it means that Germany does not know this institution and you will need to do some additional bureaucracy. For further questions, ask here and I will answer asap. =)
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.