That answer can be a bit difficult to explain. I remember seeing old footage from WDW and Disneyland from 1988 that still used the 1978 heads but sometimes showed the then new heads which started rolling out sometime in late 1988 (If I remember correctly WDW) but 1989 was more noticeable since you no longer see the previous heads.
I can tell this since the Mickey's 60th Birthday Parade from 1989 contained the Jack Wagner voice for Mickey and Minnie which was common from there previous designs. But that would also be the last time they were heard since the show Minnie's Birthday Surprise was the earliest use of Wayne and Russi Taylor's Mickey and Minnie voices.
And yes Jack Wagner voiced various Disney characters at WDW and DL and some Disney specials back in the 80's including Minnie (Which was speed up).
That's what I think, any head rollout for any Disney character is a bit hard to explain and varies for each park.