we did a lot of paperwork from the company side as well as my side, I can't remember which forms I signed. What we (company and I) did was to go and speak with the immigration before hand and we compiled a list of documents needed. This was done well in advance. After we gathered all the docs we went back to immigration for them to look at these docs. After they okayed it we terminated the work permit and did the change. Goodluck.
I did this conversion in Huahin 2 years ago. It will depend on the office and the immigration officer involved. I had a good relationship with this office over an extended time which made it easier. Goodluck.