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 don't see why narrower bars would require a longer stem, unless you shoulders/elbows don't rotate like mine do.