Can I enter Thailand on a tourist visa and later switch to a non-O marriage visa?

As the advice is correct but it is a strange way and more expensive.

OR apply for the Single Entry Non Immigrant O visa based on marriage before you make your way into Thailand (2500 Thb) and apply in Thailand for the 1 year Extension of Stay (1.900 thb).

OR

Go to Thailand and get the free 30 day Visa Exempt Entry,

Go Immigration to apply for a 30 day Extension of Stay (1.900 Thb)

Go Immigration to apply for a 60 day Extension of Stay (1.900 Thb)

Go Immigration to apply for the Non Immigrant O visa based on marriage (2.000 Thb)

Go Immigration to apply for the 1 year Extension of Stay (1.900 Thb)

And I assume you are Married to Thai.

And IF added time was needed to arrange your 400,000 Thai baht in a Thai Bank account in your name only...a Family visit extension of 60 days is available.

You can also enter Thailand with the tourist visa ..gives you 60 days on arrival plus one 30 day extension = 90 days.

Yes you can enter visa exempt..30 day's..

Plus 30 day extension....

= 60 days.

During this time you can apply for the Non O as Married to Thai.

Followed by the one year extension as Married to Thai.