Getting cast as cinderella in Into the Woods with a very small mixed range?

I was recently cast as Cinderella and I'm having some doubts about my own capabilities. I was originally auditioning for the Witch, and sang a belty but lower song. They called me back but there was some weirdness with vocal callbacks and I only read, but didn't sing again. I'm obviously excited, but I'm also very nervous.

I can sing No One is Alone or Cinderella at the Grave no problem, as I have a soft and pretty upper range which works for that, but when it comes to Steps of the Palace... oh boy. My mixed range is virtually non existent and my upper range is not loud at all.

The vocal director is trying to get me to extend my belt up to that D, and thats just not happening. Help?