Believe it or not
@Lu**
, you're far from the first person who didn't bother to look at their visa when they got their passport back (for what ever reason) and when they did look they had the wrong visa type.
It is not a long shot, it's how you get the type changed INSIDE the country without having to go back and BUY a new visa from a thai consulate in another country again. Just so you know you can't go back to Penang and get them to change the type because you already used this visa to enter thailand and it's stamped USED.
The only way to get the type changed is how I outlined, so I'd say get with the place you're volunteering for and have them call the MFA office and start seeing how it can be done, OR get new documentation and pray the borders open back up before this 90 day stamp runs out that you got on this visa so you can go buy a new Non-O