1. OBD-II Scan: Check for any fault codes, even if no dash lights are showing.
2. Live Data Monitoring: Watch MAF, MAP, boost pressure, throttle position, fuel rail pressure during a highway drive.
3. Turbo System Pressure Test: Check for leaks or actuator failure.
4. Check EGR and DPF status if applicable.
Your change over from your Visa shouldn't be a problem.
BUT!
If you are of the age nearing 75 years or if you are already over the age of 75 most definitely to maintain the Thai health insurance you have now for your OA Visa. Once you are getting your O Visa you could reduce your health insurance to a very small cover hence smaller premium to pay.
Unfortunately with an OA Visa here you are forced into a Thai Health Insurance. Very expensive especially after the age of 75 years.
Sooner then later one can expect that the O Visa also would require the same, a Thai Insurance, for a yearly renewal at the Immigration.
Keep in mind: My advise is get as early as possible into a Thai Health Insurance. If you wait till you forced getting a Thai Health Insurance, especially after the age of 75 you are paying astronomical premiums just to get in!