Can I change to Non O if I entered Thailand on visa exempt? Or do I need to get 60 day tourist visa then change no Non O?

Just got grilled on my way back from Poipet Cambodia border back to Thailand.

They took a glance at my passport and immediately pointed me to the screening desk.

I m on my 3rd multiple entry in a period of 2 years on and off travel to thailand from Europe.

He said however flying in with this type of visa would be no problem but isn’t meant for land borders.

I had sufficient funds with me plus confirmed flight out and still was being very persistent of sending me back to Combodia.

After calling the misses i was allowed to pass but was advised to fly in in the future.

So i think i will skip this border as entry point in the future.