Girls' voices don't tend to change as much as boys do. That's why they've had the same Anais voice actress on Gumball when they had to change both Gumball and Darwin. I could totally see the original Helga voice actor coming back and not sounding to different. Personally, I'd love to hear Eugene voiced by an adult Ben Diskin, even though he wouldn't sound remotely the same. Probably like Numbah 2, but that would actually be funnier. I'm glad that Dan Castellaneta is still in the picture, since there were mumblings of the newest Simpsons renegotiation had an exclusivity clause (which is why Harry Shearer almost left). After all, I'd like to think he's part of the cast because Craig is related by marriage to Matt Groening, much like early Hey Arnold strips were printed in Simpsons Magazine. Anyone remember that? One issue even had what the pilot episode of Hey Arnold was condensed into a single Sunday length strip. Only with "Hello my Ragtime Gal" instead of the James Brown soundalike.I was a bit concerned about the cast being replaced, since Craig Bartlett was always into the idea of kids being played by kids. I'm for that too, sure. But I can't picture anyone but Francesca Smith doing Helga, Justin Shenkarow doing Harold, etc. I'm fine some of the others being recast (Arnold and Eugene both went through different actors multiple times, so this is nothing new). But there are those specific few characters that I can't see anyone else playing.
While I have to admit, Hey Arnold isn't my top favorite of the classic Nicktoons, it's certainly one of them. Sure, I'm more about Ren and Stimpy and Rocko, even the misunderstood and awesome "AHHHH! Real Monsters," but I think Hey Arnold is their strongest slice of life show, even more so than Doug. And I love Doug.