You can only change your flight with the company you initially booked it through. Even if you go direct to the airline they will tell you to go through the agency.
Insurance came with my bank account and I went through an agent. It's very difficult to open a bank account unless you already have a retirement visa or work visa.
It is up to immigration on the day. If you are on a tourist visa when you arrive they will look at how long you have been in Thailand in the past 12 months. If it is close to 6 months they will start asking questions and more likely to refuse entry.
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