Should I apply for a retirement visa in the US or Thailand after my work permit expires?

I'm a US citizen and currently have a work permit which expires on September 30. I want to change to a retirement visa at that point, and understand that I have to transfer the 800K into a Thai bank account 3 months in advance (I think that's clear from past postings ;-) However, I'm a bit concerned about the timing. I'm going back to the US for a visit in July. Would it be better to apply for the non-resident visa while I'm in the States, or should I do it in Thailand when I finish my job and give my work permit back? Will I need to leave the country to switch visas? Is there anything else I need to do? (Sorry if these questions have been asked already - couldn't find the answers when I searched.)