In-country visa activation
In my Hungarian passport, my tourist visa waiver (on arrival) expires on 8 March.
My Australian passport has an unstamped TR visa valid until late April.
(I wanted to enter on the actual visa, but there was a delay in processing it, so it was mailed to me after I entered Thailand)
Can I change from one visa to another in-country, or must I make a border run?
TLDR : Answer Summary
The discussion clarifies that it is not possible to change from one visa to another within Thailand; you must exit the country and re-enter with the desired visa in the correct passport. Travelers with dual nationality can only switch passports at airports, not at land borders. Participants emphasize the importance of showing both passports upon re-entry and adhering to the requirement of exiting and entering the country on the same passport.