SOLUTION: OK, I figured out this can be done in Bambu Studio, but not in the Prepare tab where filament values are loaded and applied to models. You have to go to the Device tab, select the filament, then hit reset. Not intuitive. Leaving this post up so others having difficulty finding it can also figure it out.
It’s also quite a bit frustrating that the external filament spool cannot be reset on the printer screen itself. Not everyone uses Bambu Studio. Some people print directly from Bambu Handy.