Can I convert a tourist visa to a marriage visa in Thailand after entering on a 30-day stamp?

okay you are getting the cart WAY before the horse. There is a whole process that you need to do so you can even go register a marriage to a thai in thailand and all that and the registration needs to be done BEFORE start applying for the 90 day Non-O visa and then the year extension.

I would say you will NOT have time to pull it off comin' in visa exempt on a free 30 day stamp SO get a single entry tourist visa from the thai conuslate in your country before you come here.

Then you need to to go your embassy, get the affirmation of freedom to marry notary document, get that and the data page of your passport translated by a service, get those translations certified by the Ministry of Foreign Affairs and THEN you can go register the marriage at one of the local Amphurs (city halls)

It's only AFTER you do that that you can apply for the initial 90 day Non-O visa based on marriage to a thai, for 2000 baht at the immigration office AND at most offices you need a minimum of 15 days left on your current stamp to apply (which is why I said get a 60 day tourist visa before you come so you have plenty of time)

The above information I shared is definitely outside the scope of the visa advice group.

BUT until you get that marriage registered (and get your bank account open and the funds put in) you're not gonna get ANY visa inside the country based on marriage

Be well prepared to get married, then get your visa. Plan well. You will need certain time to complete everything.

It's not that simple

I am doing the same right now and its not so easy and we have been married for 3 years in the US good luck try anything but the 30 days that I have

It might be best to enter on a 60 day TV and then use that to apply for a non o (based on marriage) visa.

Also depends on the immigration office if they accept the in-country Non-O based on marriage to a Thai national from a newly-wed foreigner. Some offices don't accept until a certain period passes from the marriage registration. As Robert says, contact the local immigration office where you are gonna live.

That all depends on where you are inside Thailand. Not every Immigration Offices is allowed to perform a conversion from a visa exempt entry into a Non Immigrant O visa.

Best is to directly contact your local Immigration Office, is needed anyway to register your address, and ask for the hand out of requirements if they can do this conversion.