Does the 90-day reporting requirement start immediately after switching to a Non-Immigrant B visa in Thailand?

nope that's totally NOT correct at all

there is NO 90 day reporting on a 30 day visa exempt or a 30 day extension from that OR the initial 90 day Non-B visa that you haven't evidently gotten yet.

In fact their first 90 day report won't even start counting down until they get an extension of stay FROM that 90 day Non-B visa (that they don't have yet)

So get any idea of doing a 90 day report out of your head until they go to get their first extension FROM their 90 day Non-B visa. That's when the immigration office is going to put a small paper in their passport telling them when the first 90 day report is due

The paper will look like this