I don't think you should be concerned at all, they've done a great job with casting so far, and Danny has a great voice, based off a few videos I found of him. He's the new standby, so I don't think he's gone on yet, but if they cast him as well as they did the previous standby and the Lovett standby, it should still be enjoyable. Go see it just for the production and Carmello, and you'll be totally fine. Panaro is great, but his presence doesn't make or break the show.
And just a general Sweeney 3.0 report, I don't think I've posted previously but I won lottery seats a few weeks ago. TodayTix actually called me to give me the ticket offer, I guess I was the first name on their "loser list" and a winner called to decline their tickets.
The new cast is fantastic, the standouts being Panaro, John Rapson, and Eryn Lecroy. The only one I didn't love is the new Turpin, but his voice was gorgeous, so I could more or less forgive acting that wasn't my favorite. Panaro still needs a bit of time to find the role, but that's not unusual for him. He's very, very good, and a favorite Sweeney of mine, I can just see him growing into it and being even better than now. He's suited to long runs and experimenting until he finds the right version of the character for himself. And even though I prefer the role sung by a baritone, his low end is great and he sounds fantastic.
And speaking of voices, the voice Lecroy has absolutely has to be one of the strongest voices I've heard in a long time, she's loud as hell, and cuts above everyone else when needed with no struggle at all. She's great in the role, and I hope she sticks around a while.