When do I need to start my 90-day report after converting to a 90-day Non-O visa in Thailand?

You're mixed up a bit on how it works for 90 day reporting. 😮

You have to be on the SAME entry/visa/extension for 90 days to begin the reporting cycle

IF you just got the 90 day Non-O visa issued you haven't been in country on that visa long enough to do a 90 day report..

When you apply for the year extension (at C/W you can do it when you have 45 days or less left on your current stamp) they will put a paper in your passport that tells you when your FIRST 90 day report is due

The paper will look like this