When do I need to start the 90-day reporting after changing to a Non-O retirement visa?

Exactly so.

there is no "under consideration period" while you wait to do your one year extension 😕

When you apply for the in country Non-O visa you apply, pay and get a receipt that tells you to come back in 2 weeks to get the Non-O visa and new 90 day stamp inked in

BUT

Once you do that, you're not under any consideration you're here on a 90 day Non-O visa stamp and you just burn down 2 months until you have 30 days or less left on that 90 day stamp, meet the requirements and apply for the year

It's ONCE you apply for the new year extension of stay that the 90 day clock starts counting down, you don't have to worry about it until you get your year extension granted. The immigration office usually staples a small paper in your passport telling you when your first 90 day report is due when you get the year extension inked in

like this