You should apply for and receive a single entry non immigrant O visa prior to arrival rather than depend on change of a tourist visa if you have the choice. Although they may change a tourist visa it is not something that is encouraged and means more work for everyone. Rather than request tourist visa request non immigrant O visa single entry as you plan to apply for long stay after arrival (check out of Thailand).

reply from Lopburi3 to a question about retirement on another post

Does this mean that you can't leave the country and return, say a week trip to Laos, while on the non imm single entry O visa?

If this is so can a multiple entry non imm O visa be obtained in country of origin and is there any difference in requirements?

Once a non imm O visa is changed to OA on retirement grounds is there any restriction on travel outside the country?

Any travel will require a re entry permit first or you lose your stay be it 90 days single entry or a one year extension or stay.

A multi entry can sometimes be obtained and that would not require a re entry permit and you have unlimited entries. You would just get another 90 day stay when you returned.

The O visa is not changed to O-A. Your stay is extended. And yes, you must have a re entry permit for any travel.

The O-A is issued prior to travel to Thailand, in your home country, and gets a one year stamp when you enter and seems to be treated as multi entry for the first year.