Like I said, unless your arms don't rotate like mine (or most humans) I don't see why stem length would change. Pretend hold 46cm bars, now rotate your forearms to pretend hold 36cm bars. See? Torso doesn't move.
I changed from 42 to 38 and didn't need to change stem.
I like to keep my torso in the same position without my elbows compensating / my torso changing position?
Think going from from a Towel Rack to a 3T Scatto, would you really keep the same stem?