Can I change my multiple entry Tourist Visa (METV) to a Non-Immigrant ED Visa in Thailand?

Dears,

I need your expertise and experience as I have to change my multiple entry Tourist Visa METV (last entry to TH 09.04.19, next leaving until 07.06.19, in general valid until 20.08.2019) into a non-immigrant-visa category „ED“ because I am going to do an internship in a company here in Thailand starting on 01.06.19.

The documents I already have:

- Application for Change of Visa (TM.86)

- Copy of passport

- Copy of departure card

- Invitation letter/ visa request of the internship company

- Signed internship contract

- Chamber registration of company

- Copy of license to perform duty as manager (from company's director who signed the documents)

- Student confirmation from my university in Germany

- Certificate of studies with grades (from my University in Germany)

- Bank account information

Do I need further documents?

Is it possible to convert in at the immigration in Thailand?

(On the web and on the phone they stated yes, but for any reason one immigration officer advise not possible without giving a specific explanation. Should I just try again or what is the easiest way? I would prefer doing the conversion in Thailand.)

If nof possible, what is the best place to go outside Thailand? Do I have to cancel my tourist visa in advance and how?

Thank you so much for your help & advice