Evans and Downey are unlikely to return despite their characters likely appearing.

Also, the 23 episodes thing hasn't been confirmed and is unlikely. Feige made a vague statement that was just interpreted by some as 23 episodes.

Considering they brought back the other actors, I doubt they would recast someone’s voice just for this show. I don’t think they will have him in here, but he could be a surprise saved for later.

I think since we know Steve Rogers is in this show but there has been no discussion of Chris Evans returning, and Evans has said he's done with the role, that they are almost certainly getting someone to do an Evans impression for Steve, so if they're willing to do that I reckon there's a very good chance they'll get someone to do a Downey impression for Tony.

I wouldn't mind having all actors recast for What If...?. Its voice-acting we are talking about... I don't think we will need a real Chadwick Boseman, for example, to make the impression of Black Panther.

He's probably going to return.

Chadwick Boseman is confirmed for What If...?

What if Spider-Man was iron-man

Ummm.... Good idea butttt I heard that RDJ is returning.

Idk if it's just a rumour.

Maybe RDJ just ruled out future films, but said nothing about voice work for a streaming service show.