What should I consider when renewing my passport while holding a multi-entry OA visa in Thailand?

Question on expiring passports and extension of stay.

I have an OA visa and I am on an Extension of Stay to October 2025 (multi-entry). I do the extension in early September each year.

My (Australian) passport will expire in January 2026.

I have read various posts about this and it seems to me that the simplest approach is to get a new passport before attempting the extension.

I think I can renew the passport in Thailand, but this seems quite a lengthy process.

As an alternative, I have no problem with renewing my passport in Australia mid-year prior to doing the extension in Thailand in September. But that will mean that I leave Thailand on the old passport, and return with the new.

As I understand it, I simply show both passports on return and get the existing extension moved to the new passport once I have arrived.

What is unclear to me is

(a) I have an existing multi-entry stamp in my old passport. Is this still valid (and thus allowing re-entry)?

(b) Once arrived back in Thailand, do I need to immediately "transfer the stamps", or can I just wait until I do the extension, (showing both passports + associated paperwork/insurance/funds etc), as the new extension will go straight into the new passport.

Looking for advice on flaws in my logic/understanding.

Thanks all.