What is the process to change from a work permit visa (instructor at a university) to a retirement visa?

Well for starters, you need to have X amount in the Bank...then follow the paper trail.

You cannot change visas but i suspect you are on an extension of stay based upon teaching not a visa.

If you can meet the financial requirements for an extension of stay based upon retirement you can change the reason for your extension from teaching to it.

You will need a letter from the university stating the date you will no longer be working. On or a few days before that date you can go to immigration and have your current extension cancelled and then apply for the extension based upon retirement.

In November 2009, 7 years ago, I was on an extension of stay based on work with a valid work permit. I changed jobs so the extension of stay was effectively invalid on the day that I left work. I immediately applied for and received a 60 day extension based on being married to a Thai and during that 60 days obtained a new work permit for the duration of the one year contract. During the last 30 days of my 60 day extension I applied for a 1 year extension based on marriage which was denied because I did not have the necessary Kop Ror 2 form. But the income affidavit from the US Embassy stated that I made over 65000 baht a month and the immigration officer advised that I could either come back with the Kor Ror 2 form or she could immediately issue me a new 1 year extension based on retirement. I told her that I was working and had a valid work permit and she said that immigration didn't care if I was working on the retirement extension and that if I had problems with the labor office when the work permit had to be renewed then I could come back and apply for another extension for marriage as long as I had the proper paperwork. So I took the retirement extension and effectively work on the retirement extension for 10 months with a valid work permit. I don't know what would have happened if I tried to renew the permit as I packed it in before the end of the contract. This all took place in Korat right at the time that they were moving the offices from the TOT building on highway 2 in Korat to the new offices on highway 24 in Chok Chai.