Quite a few years ago I had to renew via Hong Kong, but I am aware that the system has changed. Taking at least 10 weeks to get a new passport apaprently.
It has been a while, but last time I did this, when I received the new passport in the post I just took old and new to the local Immigration where they filled in the first two pages of the new passport with details from the old. There were no documents from the embassy.
My friend came back with a list from the local immigration office, which says 3 months before. A problem for him as he only has 2 months. Worthy of note that he has previously renewed his visa through an agent at Jomtien. By looking at his passport, the Korat immigration could tell this apparently. They knew that previously he had paid to get around the problems of a lump sum.