Macleod's debut will see Milhouse in a minimized role, as the character only has one line in the episode — and will appear in a scene where he, Bart, and Lisa are all adults, so all of their voices will probably sound a little different from their usual adolescent selves.
actually pretty typical for a recurring character like this. you start small and outside of regular context and keep expanding so the weekly audience can acclimate to the change
Amazing World of Gumball pulled it off multiple times with switching to entirely new main cast the next episode after introducing new voices in a small way in an episode. So it's possible to do on a smaller time-frame as well in a two episode jump.
I guess I find it odd compared to Jimbo Jones, who Hayden also voiced. Mo Collins took over and it was a regular episode with Jimbo having several lines.
Maybe it's because Collins did an amazing job... But if that's the case find a better replacement for Milhouse.
I think it'd be perfectly normal if they just didn't write around it and that was the amount of Millhouse that was planned in the episode regardless of the voice actor.
That's what I'd prefer and that's how they (seemingly) handled it with Jimbo Jones. Mo Collins replaced Pamela Hayden, he had a small but significant role in the episode with a handful of lines. Done & done.
Giving the new voice actor a single line as older Milhouse seems weird.