Ah OK, hopefully there is not short time limit on it, then it's get it translated and off to MFA to be certified. Then your amphur will tell you what else they want, but ought not be onerous.
Probably best to start with you embassy website, most of them have details of what you need to do. For certain you will need some document from them to prove that you are eligible to marry. From there on it's easy.
Assuming you'll retain your old passport just re-enter presenting both old and new at immigration. They will add some notes to your entry in the new passport. Then go to your immigration office to get the extension transferred - if or not they will transfer the re-entry permit I'm not certain, but no big deal, it's only 1,000 for new one for the October trip if needed.
I know numerous people here on retirement extensions for well over 20 years, I've never heard any of them having serious issues. But most do their own extensions rather than using agents. Oh, and you don't need "permission" to leave and re-enter, you simply have to buy the appropriate re-entry permit.
It's a month ahead and sometimes appointments appear randomly (cancellations?) or on every new day a month ahead. Gives us something to do each morning from time to time.
Well the plan worked, brilliant move by the Government. Woke up this morning to fresh air and cool winds, well done, pat yourselves on the back. Note to self, avoid BTS and MRT this week, will be packed solid.