Things change.

When I had my first sleep study, I had an RDI of 63. I never heard the term AHI until recently. How do these terms differ?

You already know that the AHI is the per hour average of the apneas (both central and obstructive) plus the hyponeas.

Think of the RDI....Respiratory Disturbance Index (index being average per hour again) as the AHI plus other respiratory related events that don't fall into the Apnea or hyponea category for some reason or other. They are still counted and matter because they mess with sleep but they don't meet the criteria to earn an apnea or hyponea flag.