He wants to discuss some changes that have been made to the planned performance, I assume because he's using a harpsichord instead of a piano. This could be changes in tempo, dynamics, or really any aspect of the music.
"A little rusty" is him. It's used to excuse a sub-par performance.
"Mary Jo was thrilled when she beat the city's tennis champ. But Sue said it didn't mean anything because the champ was a little rusty."
"A little rusty? I'm sure he was. He won the championship in 1972."