After all this time, will they even be able to have the original cast back?

I remember reading an interview or watching a video around Season 2 releasing (might've been a few days to a week or two after Season 2 released (can't exactly remember since it's been a few years now since Season 2 released)) that mentioned the kids getting older would not cause any problems. They'd always adjust the characters' ages to match the kids' real life ages. So no need to change the focus or do any recasting. Plus if they really have to, they can just make the kids be 17-19 age range for the remainder of the show. It's still pretty common for actors and actresses that are 30-35 years old to be cast as juniors or seniors in high school. So you are looking at about 10-15 years for the kid actors (mainly the ones that play Mike, Eleven, Dustin, Lucas, Will, and Max) to still pass as high school kids. And it definitely shouldn't be a problem if they are just doing five seasons. The only thing they never talked about was if they are keeping things in the 80's or eventually moving into the 90's. So if they want to keep things in the 80's, it's possible they may have to do some SORAS action.

But everyone IS back for Season 4. There is behind-the-scenes pics and footage of the whole cast at the Season 4 table read. That happens right before filming/production starts. And it took place before all the production/filming delays hit. So they got the behind-the-scenes footage and pics out to the public before the virus really blew up. All that stuff has been on the internet too since about January-March 2020. So well over a year now. The actors playing Mike, Eleven, Dustin, Lucas, Will, and Max are all back. Oh and there is behind-the-scenes set pics too that have been releasing slowly over the last year. Everyone can be seen filming stuff for Season 4. That means no change of focus character wise. We'll still be following the same cast from Season 1 and the same characters from Season 1. No altering needed.