Can I re-enter Thailand with a new international health insurance policy while on a Non-O-A visa?

The visa is valid for one year from the date of issue You need to re-enter before that 1-year date expires, effectively stretching it to 2 years. If you plan to travel in and out of Thailand after that expiry date then you must buy a reentry permit.

Hanse Merkur Versicherung ( German insurance ) was willing to sign the foreign insurance certificate when I first used the O-A Visa .(Later I switched to O Visa.)

For a Thailand Non-Immigrant "O-A" visa (retirement), insurance is a mandatory requirement. Applicants must have a health insurance policy with a minimum coverage of 100,000 USD (or 3,000,000 Baht) for the entire period of stay, and it must include COVID-19 treatment coverage. This insurance must be issued by either a Thai or a foreign insurance company and be accompanied by a Foreign Insurance Certificate for Alien to apply for Non-Immigrant Visa Type O-A, signed with an approval stamp.

Here's a more detailed breakdown:

Minimum Coverage:

The insurance policy must cover at least 100,000 USD (or 3,000,000 Baht) for medical expenses.

COVID-19 Coverage:

The insurance must specifically cover the treatment of COVID-19.

Duration:

The insurance must cover the entire duration of the stay in Thailand, which is typically one year for an O-A visa.

Insurance Certificate:

A Foreign Insurance Certificate for Alien, signed and stamped by the insurance company, is required as a supporting document for the visa application.

Thai or Foreign Insurance:

The insurance can be issued by either a Thai insurance company or a foreign insurance company.

For more information and a list of Thai insurance companies participating in the scheme, you can visit the Thai General Insurance Association website.

Non-Immigrant O-A (Long-stay) - สถานเอกอัครราชทูต ณ กรุง ...

For medical insurance documents, the applicant must meet one of the following requirements: 7.1 The holder of a health insurance policy, issued by a Thai insura...

สถานเอกอัครราชทูต ณ กรุงออสโล

No, you re-enter before the expiry of your initial Non-OA Visa (not your entry stamp) with new 12 month insurance to get another year entry stamp

Normally it would need to be one of the approved policies with from the list of Thai companies. But you say the policy worked to enter originally back in 2024, so it might work again now? The immigration officers are likely not experts at checking insurance policies, so it's hard to say.