To make it as simple as I can, Caroll's perfectly OK with Eric playing Oscar sometimes...and eventually passing the torch to Eric just as he was fine with Matt Vogel being Big Bird.
I saw Caroll and Debi at a convention last year and Caroll remarked how great a job Eric was doing with Oscar on another TV appearance that just aired. I was last in line and there was a lull in activity, so Caroll & I got to chat quite a bit.
Sometimes, Caroll can't perform his characters (puppetry and voice) due to a schedule conflict. Then Eric and Matt will do them. There was a Big Bird TV appearance the same time the convention was going on. So, Matt did Big Bird on the TV appearance and Caroll signed autographs and met fans elsewhere.
So, bottom line: Eric (and Matt) have the official seal of approval from Caroll to perform and continue his characters. Caroll's cool with it, so that's good enough for me. And yes, it's his choice and Caroll still has a "nest" at Sesame (even if it's just off camera).