CBR contains a transcript of an interview with Josh Grelle, posted back in December 2020. In this interview, Josh pointed out that when the Attack on Titan anime was being casted and "filmed", the future of the manga at the time was unknown and the director was accounting for various outcomes
One interesting detail about Armin is that his Japanese voice actor is a woman. Does that at all color your approach to voicing him, Josh?
JG: Yes and no. I mean, Mike McFarland when he was casting the show, obviously he knew that Armin was voiced by a female, and that was a really big decision for him to make on whether he wanted to go with a female or a male. I think ultimately he ended up going with a male because there was so much potential for the show. At that time we didn't know where it was gonna go if there was ever going to be a time skip or anything like that. But just because of the sheer possibility of being with these characters for a while and having Armin eventually potentially grow up into a man, he wanted to have someone's voice that could cover the full age range. And that's one thing that at least I'm able to do.
(Emphasis my own)