If you read my comment, I said I had heard that if you used an agent, you must always use one, inferring you were locked in regardless if you have the 800k or not at a later date. Don't know this to be fact. Probably just gossip.
With all due respect, of course you use an agent if you don't have 800k. Why else would you? Process is quite simple. Just do what the immigration office where you live tells you, and be done with it. Not rocket science. Mr. Kamdee went into great detail what needs to be, however one never knows what an IO is going to tell you. Remember where we are. TIT.
John Patton. I am an American. Been retired for 9 years. Things are not always the same with each immigration office, so check the one where you intend to live. Don't recommend using an agent. Some say if you use an agent, you must always use one. Don't know this to be true or not. Easy to do yourself. Get a Non-O visa, Not Non-OA, as the OA requires insurance, which can become a tad pricey the older you get. You can apply for the visa through Thai embassy in the states. I got mine in Seoul Korea. Dropped paperwork off one day, next day picked up the visa. Cose was $80. My wife was with me. Visa was 90 days. Flew into Thailand 16 Nov2015. Visa was good till 14 Feb 2016. Went to immigration and they told me to come back when I had 30 days left on the visa and apply for a one year extension to stay. That is what I did, and have been doing annually every year since. Never an issue. Used the same IO every year. Give her a little coffee money. Cost of one year extension is 1900 baht. If you want a multiple reentry, you will pay 3800 baht. Single is 1000. Very easy. Best of luck.
Unfortunately, immigration offices tend to make up their own set of rules as they go along. Best to abide by what they say, and the process will normaljy be smooth, I said normally. I always provide a little coffee money. Never any issues in 10 years. Best of luck.