It is really only a problem for people using insurance the first time when they apply for an extension. The assumption being they don't already have insurance (like I did). This being the first year that insurance was required this probably happened a lot during this past year.
I got my 3rd 1 year extension in Oct in Bangkok. My previous extension ended Nov 18th. I had my new Pacific Cross insurance policy also begin on Nov 18th. This was the first time I had to use my insurance to get an extension. In Oct the Immigration officer told me to return on Nov 18th to complete my application. Confused, I returned home and related my story to this FB group and Tod Daniels confirmed that I had to have my insurance in the database on the day I applied. Not the day my extension expired.
Immigration assumed, since this was my first time, that I didn't have insurance already (I did). They never looked in the database. They just told me to return on Nov 18th. They made a mistake there. I made a mistake not understanding that I needed to have insurance the day I applied so I didn't insist on them looking at the database. I returned 2 days later with the insurance certificates for my current active insurance and the new policy that began on Nov 18th. They then processed my application.
This may be a case of different offices applying different rules or my particular Immigration officer.
You can apply for your 1 year extension of stay in Bangkok 45 days early. Take advantage of being able to go early to give yourself time to sort out any problems that arise. However, since you are getting insurance for the first time you may lose days/weeks from your year because they will only extend until the end of your insurance.
Be aware that you must have the health insurance active and in the database on the day you apply, not the day your visa expires or the last day of your permission to stay.
A year from now, when you apply for you first 1 year extension you will need to meet the insurance requirement. There is plenty of time to look into this but it is something to put on your calendar to do before the year is up.
The problem, as usual, is that the OP did not state what Immigration office they are using. They then get a lot of conflicting advice. My point is that as you get this varied advice to go to Immigration over prepared if it costs you little or nothing. Take notes after you have done it the first time.
No one ever complains because they brought too much documentation.
In addition to the bank letter and statement you should make a small transaction and update your bank book the day you apply. Also your bank letter should be less than a week old when you apply.
Which is why I suggested that he ask his bank at this time. A while back the info was that only the large branches could produce a 1 year statement. I was able to get my statement from a small branch and I would encourage people to at least try their nearest most convenient branch at this time.