Hey Jonathan. I think you are somehwta mistaken about the original Vernon routine. The third ace that turns over in the routine happens after turning over the second card (straight out, not as an effect), then the cards counted through and you show the third ace is face up.
I think you are referring to the SECOND ace that turns over in the routine. The first one is shown as the only face up one using an elmsley, and immediately you follow with another Elmsley to show that the second ace is the only one face up. In other words, the first ace changes to the second ace, as you say. I do not present it that way. I keep it cosistent with the theme that the first ace turns face dwon and the second one turns face up, all at once.
Anyway, I do not know if anyone specific takes credit for this "change" as it is not so much of a change, just the genius of the Elmsley count and how it is used. I guess Vernon came up with it...