Can I change my holiday visa to a Non-O retirement visa while in Thailand?

Why are you going on a tourist visa if you really want a Non-O visa? You're paying two sets of fees for one trip.

I almost made the same choice that you're planning until I realized that the new online process is much simpler/faster and doesn't require mailing your passport. Search youtube for 'letslivethailand thai retirement visa in minutes' to see how easy it is. It changed my mind (I also have a thai bank account so I also thought incountry was best - NOPE).

Neither is super expensive. But it's much better if you get the non-o before you arrive. Just make sure you enter that visa number on the TDAC and ignore the tourist visa.

Your situation is almost the same as mine i will share what I did back in 2022 I came in on a visa exempt (30days) I already have bank acct with funds before covid came with more than 15days of my entry i apply for 90days nonO in CW bkk with TM87 (change visa type), my bank statements and lease agreement payed 2000baht issued with a recept and returning date which is 1 or 2 days before my actual stay expired returned to immigration office and got the 90days nonO stamp on my passport....run it down to less than 30days and apply for the 1yr extensions (TM7) and almost identical sets of documents hope this helps

the problem could arise from the demand of the police order, that if you want to change from a touristic entry to an initial 90-days Non-imm-O visa, you need proof of 800,000 THB minimum in your Thai bank account (no seasoning required at this point) and you need a proof that the money has come from abroad . . . . .you should ask "your" Immigration on how they see it

ah okay, you already have the bank account. The problem could be, that for a "change of visa type" from a touristic entry to a 90-days Non-O visa inside Thailand, you will need to prove the money came from abroad (only in Pattaya they have different rules) and you need a rental contract or something alike

While you can do this and covert a tourist entry to a Non-O inside Thailand a number of immigration offices require you prove that the money has come from overseas. It would be much simpler to get the 90 day Non-O visa before coming to Thailand and then use the funds in your thai bank for the extension of stay which doesn't require the funds come from overseas

get a 90-days Non-Imm-O Retirement visa before you fly to Thailand, because on a touristic entry you can't get a bank account opened (which you will need) unless you shovel 2000 US Dollars to an agent . . . . PS: sorry I missed he already has a Thai bank account

As mentioned

Much easier path if you apply for a

Non-O retirement Visa Single entry 90 days - then once here - get the 12 month extension

Just get Non O before less stressful and more time to sort everything out

Why waste time and money on a tourist visa?

Yep as long as you have the funds in a Thai bank account. Go to your local immigration office and get the handout of required documents. Not all offices may not offer this service.

Get the Non O before you come is the usual recommendation