Good point. The concert isn't until August, and she hasn't done any lives since the anniversary special. Assuming she's been doing voice lessons and training all this time, would that possibly be enough time for her?I doubt it'll be near empty. I would be more worried about her performance during the concert. If you're singing with an orchestra you better have great vocal skills, otherwise you're just embarrassing yourself and wasting the orchestra's time.
Now, I listened to recordings of their anniversary live, and based on that she's not ready. If she's been taking intensive vocal lessons, master classes, whatever since then, then she might pull it off.
Another thing is that the announcement mentions "an elegant world of music ". This makes me think she'll be performing bel canto repertoire.