I noticed that after every season, Veronica's pitch would change slightly. Orange Islands - it went from sounding deep in Kanto, to sounding REALLY high in Orange Islands (the voice was high, but Ash still sounded like a boy, in my opinion)
Johto Journeys - same voice as Kanto, except slightly higher (I like this pitch best, I think it fits Ash perfectly, relecting his energetic personality) =D
Johto League champions - same voice as Johto Journeys, pretty much, I didn't notice any change in inflection
Master Quest - the voice is more polished overall, and it's less rough/stuffy than the Johto Journeys voice sounds (I like this pitch too! It's awesome!)
Advanced, Advanced Challenge, and Advanced Battle - same voice as Master Quest, except the pitch is deeper =)
I think if TPCi had allowed Veronica to stay and voice Ash, she would've used her Hoenn Ash voice for the Battle Frontier and D/P seasons.
Personally, I think Sarah Natochenny is a fine VA. I have nothing against her. She's doing a great job in the Sinnoh League episodes. But, no matter how good Sarah gets, she just isn't Ash to me. I grew up watching the old episodes of Pokemon, and that's why I'll always prefer Veronica Taylor as Ash.
But, that's just my opinion. XD