The process I went through yesterday, to convert to the 3.5M policy, consisted of these steps:
I signed the Change of Coverage form, on which I selected 3.5 M Limit, Plan 3 (100,000) deductible;
I signed the Renewal Notice form, renewing the policy I've had for the past 2 years;
I paid the premium of 36,700 baht;
I submitted the Change of Coverage form, the Renewal Notice form, a copy of my passport, and the payment receipt via e-mail to the insurance company;
I received a response from the insurance company stating they will send a Health Certificate, and an E-Policy within 2 working days by e-mail; and they will send a Health Care Card by snail mail within 15 working days.
After I receive the Health Certificate and E-Policy, I will query the insurance company whether they have updated the TGIA database with the new information so I can process my extension through Immigration.