You pay for the full year. The rate has gone down a little because I haven’t made a claim. Essentially I’m self insuring for the first 40,000 baht and I’ve never spent that much in any year.
I have Pacific Cross that I first purchased in 2018 before there was an OA insurance requirement. It is a 10 million baht policy with a 40,000 baht deductible. It easily qualified for the OA insurance requirement and I then used it for the LTR.
I had an OA for 6 years. I have a 10 yr pensioner LTR visa now. If you meet the $80,000/yr passive income requirement then you might want to consider an LTR.
You can’t change from an OA to an O inside Thailand. After the first year of your OA you have to leave Thailand and then return as a tourist and then apply for the non-O and 90 days later apply for the one year extension. You can always buy a single or multiple re-entry permit at Immigration. The most important thing these days is to get your Thai bank account set up during that first year.